chore(deps): update act to v0.261.8 (#791)

## Summary
- Update `gitea.com/gitea/act` from pseudo-version `v0.261.7-0.20251202193638-5417d3ac6742` to `v0.261.8`
- Update yaml import in `workflow.go` from `gopkg.in/yaml.v3` to `go.yaml.in/yaml/v4` to match act's yaml v4 migration
- Add tests to verify yaml/v4 upgrade works correctly

## Changes included in act v0.261.8
- Recover from panics in parallel executor workers ([gitea/act#153](https://gitea.com/gitea/act/pulls/153))
- Fix max-parallel support for matrix jobs ([gitea/act#150](https://gitea.com/gitea/act/pulls/150))
- Fix yaml with prefixed newline broken setjob + yaml v4 ([gitea/act#144](https://gitea.com/gitea/act/pulls/144))
- Fixed typo ([gitea/act#151](https://gitea.com/gitea/act/pulls/151))

## Tests added
- **`Test_generateWorkflow`**: 7 new cases (was 1), covering: no needs, needs as list, workflow env/triggers, container+services, matrix strategy, special YAML characters, and invalid YAML error handling
- **`Test_yamlV4NodeRoundTrip`**: Directly exercises `go.yaml.in/yaml/v4` — `yaml.Node` construction, marshal/unmarshal round-trip, and node kind constants

Fixes: https://gitea.com/gitea/act_runner/issues/371
Fixes: https://gitea.com/gitea/act_runner/issues/772
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/791
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

go.mod

@@ -21,6 +21,8 @@ require (
 	gotest.tools/v3 v3.5.1
 )
 
+require go.yaml.in/yaml/v4 v4.0.0-rc.2
+
 require (
 	cyphar.com/go-pathrs v0.2.1 // indirect
 	dario.cat/mergo v1.0.0 // indirect
@@ -100,7 +102,7 @@ require (
 	gopkg.in/yaml.v2 v2.4.0 // indirect
 )
 
-replace github.com/nektos/act => gitea.com/gitea/act v0.261.7-0.20251202193638-5417d3ac6742
+replace github.com/nektos/act => gitea.com/gitea/act v0.261.8
 
 replace github.com/go-git/go-git/v5 => github.com/go-git/go-git/v5 v5.16.2