Fix regex for GITHUB_ENV parsing

[catthehacker]

While troubleshooting cachix/install-nix-action (cachix/install-nix-action#111) we hit a bug where envvars in $GITHUB_ENV file that contain equals sign (=) do not work
This PR fixes that issue

[codecov]

Codecov Report

Merging #893 (3d31b76) into master (0f04942) will increase coverage by 6.84%.
The diff coverage is 60.99%.

@@            Coverage Diff             @@
##           master     #893      +/-   ##
==========================================
+ Coverage   49.27%   56.11%   +6.84%     
==========================================
  Files          23       25       +2     
  Lines        2401     3949    +1548     
==========================================
+ Hits         1183     2216    +1033     
- Misses       1090     1531     +441     
- Partials      128      202      +74     

Impacted Files	Coverage Δ
pkg/common/outbound_ip.go	0.00% <0.00%> (ø)
pkg/container/docker_volume.go	0.00% <0.00%> (ø)
pkg/model/action.go	0.00% <ø> (ø)
pkg/container/docker_run.go	5.53% <14.54%> (+3.60%)	⬆️
pkg/common/git.go	49.82% <31.81%> (-9.97%)	⬇️
pkg/model/planner.go	49.78% <42.50%> (+16.70%)	⬆️
pkg/container/docker_build.go	58.97% <50.00%> (+58.97%)	⬆️
pkg/container/docker_pull.go	28.35% <58.06%> (+10.17%)	⬆️
pkg/model/workflow.go	52.14% <63.63%> (+26.42%)	⬆️
pkg/artifacts/server.go	67.32% <67.32%> (ø)
... and 31 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4ae71b5...3d31b76. Read the comment docs.