test: prevent data race detection in TestActionCache (#858)

Data race is being flagged because a goroutine is currently writing tar data into `atar`, and `assert.NotEmpty(atar)` performs internal structure reflection into the returned `io.PipeWriter` and violates its internal synchronization primitives.  This assertion doesn't seem to add any value to the test compared to just reading the pipe, so it has been removed.

Data race details (abbreviated):
```
==================
WARNING: DATA RACE
Write at 0x00c00041fbc0 by goroutine 55:
  sync/atomic.CompareAndSwapInt32()
      /home/mfenniak/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.linux-amd64/src/runtime/race_amd64.s:361
+0xb
  sync/atomic.CompareAndSwapInt32()
      <autogenerated>:1 +0x18
  sync.(*Mutex).Lock()
      /home/mfenniak/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.linux-amd64/src/sync/mutex.go:46
+0x28
  io.(*pipe).write()
      /home/mfenniak/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.linux-amd64/src/io/pipe.go:81
+0x9e
  io.(*PipeWriter).Write()
      /home/mfenniak/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.linux-amd64/src/io/pipe.go:161
+0x46
  archive/tar.(*Writer).Flush()
      /home/mfenniak/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.linux-amd64/src/archive/tar/writer.go:59
+0xcc
  archive/tar.(*Writer).WriteHeader()
      /home/mfenniak/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.linux-amd64/src/archive/tar/writer.go:71
+0x46
  code.forgejo.org/forgejo/runner/v9/act/runner.actionCacheCopyFileOrDir()
      /home/mfenniak/Dev/forgejo-runner/act/runner/action_cache.go:203
+0xabd
  code.forgejo.org/forgejo/runner/v9/act/runner.GoGitActionCache.GetTarArchive.func2.1()
      /home/mfenniak/Dev/forgejo-runner/act/runner/action_cache.go:154
+0xa5
...

Previous read at 0x00c00041fbc0 by goroutine 9:
  reflect.typedmemmove()
      /home/mfenniak/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.linux-amd64/src/runtime/mbarrier.go:213
+0x0
  reflect.packEface()
      /home/mfenniak/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.linux-amd64/src/reflect/value.go:136
+0xc5
  reflect.valueInterface()
      /home/mfenniak/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.linux-amd64/src/reflect/value.go:1513
+0x179
  reflect.Value.Interface()
      /home/mfenniak/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.linux-amd64/src/reflect/value.go:1484
+0x106
  github.com/stretchr/testify/assert.isEmpty()
      /home/mfenniak/go/pkg/mod/github.com/stretchr/testify@v1.10.0/assert/assertions.go:735
+0x7f
  github.com/stretchr/testify/assert.NotEmpty()
      /home/mfenniak/go/pkg/mod/github.com/stretchr/testify@v1.10.0/assert/assertions.go:769
+0x56
  github.com/stretchr/testify/assert.(*Assertions).NotEmpty()
      /home/mfenniak/go/pkg/mod/github.com/stretchr/testify@v1.10.0/assert/assertion_forward.go:1175
+0xb1
  code.forgejo.org/forgejo/runner/v9/act/runner.TestActionCache.func1()
      /home/mfenniak/Dev/forgejo-runner/act/runner/action_cache_test.go:60
+0x21e
==================
```

<!--start release-notes-assistant-->
<!--URL:https://code.forgejo.org/forgejo/runner-->
- other
  - [PR](https://code.forgejo.org/forgejo/runner/pulls/858): <!--number 858 --><!--line 0 --><!--description dGVzdDogcHJldmVudCBkYXRhIHJhY2UgZGV0ZWN0aW9uIGluIFRlc3RBY3Rpb25DYWNoZSBbc2tpcCBjYXNjYWRlXQ==-->test: prevent data race detection in TestActionCache [skip cascade]<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/858
Reviewed-by: Gusted <gusted@noreply.code.forgejo.org>
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Co-committed-by: Mathieu Fenniak <mathieu@fenniak.net>

act/runner/action_cache_test.go

@@ -57,9 +57,10 @@ func TestActionCache(t *testing.T) {
 				return
 			}
 			atar, err := cache.GetTarArchive(ctx, c.CacheDir, sha, "js")
-			if !a.NoError(err) || !a.NotEmpty(atar) {
+			if !a.NoError(err) {
 				return
 			}
+			defer atar.Close()
 			mytar := tar.NewReader(atar)
 			th, err := mytar.Next()
 			if !a.NoError(err) || !a.NotEqual(0, th.Size) {