[CI] Enable Linux nightly main on PRs (#137)

### Motivation

PR CI was missing testing with nighly Linux toolchains from main.

### Modifications

Enable nightly toolchain CI as well.

### Result

More test coverage on PRs.

### Test Plan

See this PR's CI if this works or if we need more changes.

Author: czechboy0

.github/workflows/pull_request.yml

@@ -19,7 +19,7 @@ jobs:
       linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_0_arguments_override: "--explicit-target-dependency-import-check error"
       linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error"
-      linux_nightly_main_enabled: false
+      linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"
 
   integration-test:
     name: Integration test

Sources/OpenAPIRuntime/Base/Acceptable.swift

@@ -11,6 +11,7 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+import Foundation
 
 /// The protocol that all generated `AcceptableContentType` enums conform to.
 public protocol AcceptableProtocol: RawRepresentable, Sendable, Hashable, CaseIterable where RawValue == String {}

Sources/OpenAPIRuntime/Base/ContentDisposition.swift

@@ -11,6 +11,7 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+import Foundation
 
 /// A parsed representation of the `content-disposition` header described by RFC 6266 containing only
 /// the features relevant to OpenAPI multipart bodies.

Sources/OpenAPIRuntime/Interface/CurrencyTypes.swift

@@ -13,6 +13,7 @@
 //===----------------------------------------------------------------------===//
 
 import HTTPTypes
+import Foundation
 
 /// A container for request metadata already parsed and validated
 /// by the server transport.

Sources/OpenAPIRuntime/Multipart/MultipartBoundaryGenerator.swift

@@ -11,6 +11,7 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+import Foundation
 
 /// A generator of a new boundary string used by multipart messages to separate parts.
 public protocol MultipartBoundaryGenerator: Sendable {

Tests/OpenAPIRuntimeTests/Base/Test_OpenAPIValue.swift

@@ -12,8 +12,8 @@
 //
 //===----------------------------------------------------------------------===//
 import XCTest
-#if canImport(Foundation)
-@preconcurrency import Foundation
+import Foundation
+#if canImport(CoreFoundation)
 import CoreFoundation
 #endif
 @_spi(Generated) @testable import OpenAPIRuntime

Tests/OpenAPIRuntimeTests/Multipart/Test_MultipartBytesToFramesSequence.swift

@@ -14,6 +14,7 @@
 import XCTest
 @_spi(Generated) @testable import OpenAPIRuntime
 import Foundation
+import HTTPTypes
 
 final class Test_MultipartBytesToFramesSequence: Test_Runtime {
     func test() async throws {

Tests/OpenAPIRuntimeTests/Multipart/Test_MultipartFramesToBytesSequence.swift

@@ -14,6 +14,7 @@
 import XCTest
 @_spi(Generated) @testable import OpenAPIRuntime
 import Foundation
+import HTTPTypes
 
 final class Test_MultipartFramesToBytesSequence: Test_Runtime {
     func test() async throws {

Tests/OpenAPIRuntimeTests/Multipart/Test_MultipartFramesToRawPartsSequence.swift

@@ -14,6 +14,7 @@
 import XCTest
 @_spi(Generated) @testable import OpenAPIRuntime
 import Foundation
+import HTTPTypes
 
 final class Test_MultipartFramesToRawPartsSequence: Test_Runtime {
     func test() async throws {

Tests/OpenAPIRuntimeTests/Multipart/Test_MultipartRawPartsToFramesSequence.swift

@@ -14,6 +14,7 @@
 import XCTest
 @_spi(Generated) @testable import OpenAPIRuntime
 import Foundation
+import HTTPTypes
 
 final class Test_MultipartRawPartsToFramesSequence: Test_Runtime {
     func test() async throws {

Tests/OpenAPIRuntimeTests/Multipart/Test_MultipartValidationSequence.swift

@@ -14,6 +14,7 @@
 import XCTest
 @_spi(Generated) @testable import OpenAPIRuntime
 import Foundation
+import HTTPTypes
 
 final class Test_MultipartValidationSequence: Test_Runtime {
     func test() async throws {