fix(secret): use step.Environment for hydrating env in secret containers

[ecrupper]

This isn't actually a fix right now, but it is necessary for go-vela/server#1353, as the build number will not be determined until post-compilation. It also just makes a bit more sense compared to copying a portion of that Environment function.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.88%. Comparing base (ca78232) to head (f8990b1).

Files with missing lines	Patch %	Lines
executor/linux/secret.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #669      +/-   ##
==========================================
- Coverage   52.95%   52.88%   -0.08%     
==========================================
  Files         124      124              
  Lines        5342     5334       -8     
==========================================
- Hits         2829     2821       -8     
  Misses       2300     2300              
  Partials      213      213              

Files with missing lines	Coverage Δ
executor/linux/secret.go	71.97% <50.00%> (-1.36%)	⬇️

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