Export RUST_BACKTRACE=1 in --feature backtrace CI test

as this test often fails on windows which is hard to debug locally
for most contributors.

Author: TheBlueMatt

.github/workflows/build.yml

@@ -149,7 +149,7 @@ jobs:
       - name: Test backtrace-debug builds on Rust ${{ matrix.toolchain }}
         if: "matrix.toolchain == 'stable'"
         run: |
-          cd lightning && cargo test --verbose --color always --features backtrace
+          cd lightning && RUST_BACKTRACE=1 cargo test --verbose --color always --features backtrace
       - name: Test on Rust ${{ matrix.toolchain }} with net-tokio
         if: "matrix.build-net-tokio && !matrix.coverage && matrix.build-tx-sync"
         run: cargo test --verbose --color always