setup-go-toolchain: own the Windows PATH for cgo

The toolchain action's contract should be "after this action, Go
and cgo work in any shell on this runner." But on Windows we were
still asking each caller to do

    if [ "$RUNNER_OS" = "Windows" ]; then
      export PATH=$(cygpath -u "$MSYS2_LOCATION"/ucrt64/bin):"$PATH"
    fi

inline before invoking go test, plus thread MSYS2_LOCATION through
as an env var sourced from the action's msys2-location output.
That leaks msys2 implementation details into every workflow that
runs cgo builds in shell: bash on Windows.

Move the PATH addition into the action: a new "Add MSYS2 to PATH
(Windows)" step writes the cygpath-converted UCRT64 bin dir to
$GITHUB_PATH after the msys2 install. Callers using shell: bash on
Windows now find gcc and the ICU libs without ceremony. Workflows
using shell: 'msys2 {0}' (e.g. ci-bats-windows.yaml's build) are
unaffected — that shell sets up its own environment.

Drop the now-unused msys2-location output and the corresponding
env-var threading from ci-go-tests.yaml.

Author: reltuk

.github/actions/setup-go-toolchain/action.yml

@@ -1,10 +1,5 @@
 name: 'Set up Go toolchain'
-description: 'Set up Go and platform-specific dependencies (ICU4C) needed to build dolt. On macOS, sets CGO flags pointing at brew icu4c. On Windows, installs MSYS2 with icu/toolchain/pkg-config and exposes the install path via the msys2-location output. On Linux, just sets up Go.'
-
-outputs:
-  msys2-location:
-    description: 'Path to the MSYS2 installation (Windows only).'
-    value: ${{ steps.msys2.outputs.msys2-location }}
+description: 'Sets up Go and the platform-specific dependencies (ICU4C) needed to build dolt. After this action runs, Go and cgo work in `shell: bash` and `shell: msys2 {0}` on every runner OS — callers do not need to manage CGO flags or PATH themselves. macOS: sets CGO_CPPFLAGS/CGO_LDFLAGS pointing at brew icu4c. Windows: installs MSYS2 with icu/toolchain/pkg-config and prepends the UCRT64 bin dir to GITHUB_PATH so cgo can find gcc and the ICU libs.'
 
 runs:
   using: 'composite'
@@ -29,3 +24,9 @@ runs:
         path-type: inherit
         msystem: UCRT64
         pacboy: icu:p toolchain:p pkg-config:p
+    - name: Add MSYS2 to PATH (Windows)
+      if: runner.os == 'Windows'
+      shell: bash
+      run: |
+        loc='${{ steps.msys2.outputs.msys2-location }}'
+        echo "$(cygpath -u "$loc/ucrt64/bin")" >> "$GITHUB_PATH"

.github/workflows/ci-go-tests.yaml

@@ -50,16 +50,10 @@ jobs:
     steps:
       - uses: actions/checkout@v6
       - uses: ./.github/actions/setup-go-toolchain
-        id: toolchain
       - name: Run shard
         working-directory: ./go
         run: |
-          if [ "$RUNNER_OS" = "Windows" ]; then
-            export PATH=$(cygpath -u "$MSYS2_LOCATION"/ucrt64/bin):"$PATH"
-          fi
           go run ./utils/cigotests run \
             --shard "${{ matrix.shard }}" \
             --os "${{ matrix.os }}" \
             --event "${{ github.event_name }}"
-        env:
-          MSYS2_LOCATION: ${{ steps.toolchain.outputs.msys2-location }}