Update README.md: nix run command

[GuillaumeDesforges]

There is a new Nix command

https://nixos.wiki/wiki/Nix_command

it allows to run act directly without entering a shell.

[mergify]

@GuillaumeDesforges this pull request is now in conflict 😩

[GuillaumeDesforges]

Thanks @cplee 🙏

[codecov]

Codecov Report

Merging #1121 (fc45f2d) into master (4f8da0a) will increase coverage by 2.49%.
The diff coverage is 78.93%.

@@            Coverage Diff             @@
##           master    #1121      +/-   ##
==========================================
+ Coverage   57.50%   60.00%   +2.49%     
==========================================
  Files          32       39       +7     
  Lines        4594     4938     +344     
==========================================
+ Hits         2642     2963     +321     
- Misses       1729     1732       +3     
- Partials      223      243      +20     

Impacted Files	Coverage Δ
pkg/model/action.go	0.00% <ø> (ø)
pkg/model/planner.go	50.73% <ø> (+0.32%)	⬆️
pkg/model/workflow.go	50.91% <ø> (ø)
pkg/container/docker_run.go	5.11% <2.50%> (-0.43%)	⬇️
pkg/common/git.go	49.31% <28.57%> (-0.52%)	⬇️
pkg/container/file_collector.go	42.05% <42.05%> (ø)
pkg/runner/logger.go	66.00% <67.56%> (+0.56%)	⬆️
pkg/runner/runner.go	73.72% <73.68%> (-2.75%)	⬇️
pkg/exprparser/interpreter.go	74.57% <75.00%> (+1.17%)	⬆️
pkg/runner/expression.go	91.02% <78.26%> (+0.20%)	⬆️
... and 19 more

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