[skip changelog] Force windows env to use bash as shell instead of powershell

[rsora]

As per GitHub announcement the default shell for Windows steps is pwsh.

This causes the Install Go deps step fail on windows.

  Install Go deps13s
##[error]Process completed with exit code 1.
Run go get github.com/golangci/govet
  go get github.com/golangci/govet
  go get golang.org/x/lint/golint
  go get github.com/golang/protobuf/protoc-gen-go
  shell: C:\Program Files\PowerShell\6\pwsh.EXE -command ". '{0}'"
  env:
    GOROOT: C:\hostedtoolcache\windows\go\1.13.3\x64
go: finding github.com/golangci/govet latest
go: downloading github.com/golangci/govet v0.0.0-20180818181408-44ddbe260190
go: extracting github.com/golangci/govet v0.0.0-20180818181408-44ddbe260190
go: downloading golang.org/x/tools v0.0.0-20190328211700-ab21143f2384
go: extracting golang.org/x/tools v0.0.0-20190328211700-ab21143f2384
go: finding golang.org/x/tools v0.0.0-20190328211700-ab21143f2384
go: finding golang.org/x/lint latest
go: downloading golang.org/x/lint v0.0.0-20190930215403-16217165b5de
go: extracting golang.org/x/lint v0.0.0-20190930215403-16217165b5de
go: downloading golang.org/x/lint v0.0.0-20190313153728-d0100b6bd8b3
go: extracting golang.org/x/lint v0.0.0-20190313153728-d0100b6bd8b3
go: finding github.com/golang/protobuf v1.3.2
##[error]Process completed with exit code 1.

Switching to bash solves the issue as per github documentation

The default shell on non-Windows platforms with a fallback to sh. When specifying a bash shell on Windows, the bash shell included with Git for Windows is used.

[masci]

LGTM, 🍰 for the comments