Fix panic when stage's script is not wholly known

[srevinsaju]

No description provided.

[codecov]

Codecov Report

Merging #57 (f4aef85) into v2 (7fd24d4) will decrease coverage by 0.04%.
Report is 4 commits behind head on v2.
The diff coverage is 54.83%.

@@            Coverage Diff             @@
##               v2      #57      +/-   ##
==========================================
- Coverage   62.71%   62.67%   -0.04%     
==========================================
  Files         103      103              
  Lines        6691     6736      +45     
==========================================
+ Hits         4196     4222      +26     
- Misses       2221     2238      +17     
- Partials      274      276       +2     

Files	Coverage Δ
internal/ci/conductor_hcl.go	95.62% <ø> (ø)
internal/ci/stage_hooks.go	100.00% <100.00%> (ø)
internal/ci/stage_prop.go	100.00% <100.00%> (ø)
internal/runnable/options.go	100.00% <100.00%> (ø)
internal/runnable/status.go	0.00% <ø> (ø)
internal/ui/colors.go	23.68% <0.00%> (-2.79%)	⬇️
internal/ci/conductor.go	72.82% <42.10%> (-3.54%)	⬇️
internal/ci/stage_run.go	62.30% <59.37%> (-0.45%)	⬇️

... and 1 file with indirect coverage changes

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