fix: interpolate action input defaults

[KnisterPeter]

This fixes the regression to interpolate input defaults which contain
expressions.

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ EDITORCONFIG	editorconfig-checker	1		0	0.01s
✅ REPOSITORY	gitleaks	yes		no	2.92s
✅ REPOSITORY	git_diff	yes		no	0.0s
✅ REPOSITORY	secretlint	yes		no	1.42s

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[codecov]

Codecov Report

Merging #1376 (54ff401) into master (4f8da0a) will increase coverage by 6.59%.
The diff coverage is 75.40%.

@@            Coverage Diff             @@
##           master    #1376      +/-   ##
==========================================
+ Coverage   57.50%   64.10%   +6.59%     
==========================================
  Files          32       41       +9     
  Lines        4594     6383    +1789     
==========================================
+ Hits         2642     4092    +1450     
- Misses       1729     1996     +267     
- Partials      223      295      +72     

Impacted Files	Coverage Δ
pkg/model/action.go	0.00% <0.00%> (ø)
pkg/model/step_result.go	0.00% <ø> (ø)
pkg/container/docker_run.go	13.06% <12.32%> (+7.52%)	⬆️
pkg/model/planner.go	48.82% <25.00%> (-1.60%)	⬇️
pkg/model/workflow.go	50.97% <32.14%> (+0.05%)	⬆️
pkg/container/docker_pull.go	33.33% <33.33%> (ø)
pkg/container/file_collector.go	45.87% <45.87%> (ø)
pkg/common/git/git.go	50.00% <47.91%> (ø)
pkg/container/docker_auth.go	47.61% <50.00%> (+2.61%)	⬆️
pkg/exprparser/interpreter.go	73.37% <53.48%> (-0.02%)	⬇️
... and 31 more

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[cplee]

Thanks @KnisterPeter