ci: upgrade actions/setup-go to v6

Updates 8 occurrences across 4 workflow files from v5 to v6.
v6 runs on Node 24; v5 runs on Node 20 which is being
deprecated on GitHub Actions runners.

v6 defaults GOTOOLCHAIN=local, so the transform-sdk-build
integration-test build (whose tests module needs a newer Go than the
pinned GO_VERSION) gets GOTOOLCHAIN=auto to keep fetching the
toolchain as before.

See https://github.com/actions/setup-go/releases/tag/v6.0.0

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: andrewhsu

.github/workflows/lint-bazel-pkg-tool.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: stable
           cache: false

.github/workflows/rpk-build.yml

@@ -44,7 +44,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: stable
           cache-dependency-path: 'src/go/rpk/go.sum'
@@ -57,7 +57,7 @@ jobs:
     runs-on: ${{ matrix.runner }}
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: stable
           cache-dependency-path: 'src/go/rpk/go.sum'
@@ -67,7 +67,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: stable
       - uses: bufbuild/buf-action@v1

.github/workflows/transform-sdk-build.yml

@@ -33,12 +33,17 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: ${{ env.GO_VERSION }}
           cache-dependency-path: src/transform-sdk/go/transform/go.sum
       - name: Build integration tests
         working-directory: src/transform-sdk/tests
+        env:
+          # setup-go@v6 defaults GOTOOLCHAIN=local; this module's go.mod
+          # requires a newer Go than the pinned GO_VERSION, so allow the
+          # toolchain to be fetched as it was under v5.
+          GOTOOLCHAIN: auto
         run: go test -c -o ./wasm-integration-test
       - name: Upload integration test binary
         uses: actions/upload-artifact@v4
@@ -51,7 +56,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: ${{ env.GO_VERSION }}
           cache-dependency-path: src/transform-sdk/go/transform/go.sum
@@ -69,7 +74,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: ${{ env.GO_VERSION }}
           cache-dependency-path: src/transform-sdk/go/transform/go.sum

.github/workflows/transform-sdk-release.yml

@@ -19,7 +19,7 @@ jobs:
       contents: write
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: 1.22
           cache-dependency-path: src/transform-sdk/go/transform/go.sum