fix: subpath actions via new artifact cache

[ChristopherHX]

This bug is behind a feature flag, which is good so no impact for regular users of act.

readfile have always read from action.yml and didn't passed the absolute path inside the action repo. This is fixed now, so readfile applies the subpath as prefix

Additionally the repo content has been copied into the subpath, so I applied a fix to use the trimmed actionContainer path as the code used without the feature toggle

To verify

on: push
jobs:
  _:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/cache/save@v4

using vscode debug config

        {
            "name": "Launch Package",
            "type": "go",
            "request": "launch",
            "mode": "auto",
            "program": "${workspaceFolder}",
            "args": [
                "-W",
                "w.yml",
                "-P",
                "ubuntu-latest=-self-hosted",
                "--use-new-action-cache"
            ]
        }

After this fix we get

[w.yml/_] ⭐ Run Main actions/cache/save@v4
[w.yml/_]   | [warning]Key is not specified.
[w.yml/_]   ✅  Success - Main actions/cache/save@v4
[w.yml/_] Cleaning up container for job _
[w.yml/_] 🏁  Job succeeded

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ EDITORCONFIG	editorconfig-checker	2		0	0.01s
✅ REPOSITORY	gitleaks	yes		no	2.34s
✅ REPOSITORY	git_diff	yes		no	0.17s
✅ REPOSITORY	grype	yes		no	7.32s
✅ REPOSITORY	secretlint	yes		no	1.17s
✅ REPOSITORY	trivy-sbom	yes		no	0.63s
✅ REPOSITORY	trufflehog	yes		no	6.62s

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[codecov]

Codecov Report

Attention: 1029 lines in your changes are missing coverage. Please review.

Comparison is base (4989f44) 61.22% compared to head (f7c4deb) 60.96%.
Report is 308 commits behind head on master.

Files	Patch %	Lines
pkg/artifactcache/handler.go	65.32%	103 Missing and 43 partials ⚠️
pkg/runner/run_context.go	73.37%	75 Missing and 19 partials ⚠️
pkg/runner/expression.go	55.17%	66 Missing and 12 partials ⚠️
pkg/container/docker_run.go	1.47%	66 Missing and 1 partial ⚠️
pkg/runner/action_cache.go	50.74%	49 Missing and 17 partials ⚠️
pkg/container/docker_network.go	0.00%	56 Missing ⚠️
pkg/model/planner.go	28.57%	53 Missing and 2 partials ⚠️
pkg/model/workflow.go	52.72%	42 Missing and 10 partials ⚠️
pkg/runner/reusable_workflow.go	52.47%	42 Missing and 6 partials ⚠️
pkg/common/outbound_ip.go	0.00%	44 Missing ⚠️
... and 27 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2170      +/-   ##
==========================================
- Coverage   61.22%   60.96%   -0.26%     
==========================================
  Files          46       53       +7     
  Lines        7141     8998    +1857     
==========================================
+ Hits         4372     5486    +1114     
- Misses       2462     3072     +610     
- Partials      307      440     +133     

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