Apply CredentialsPlugin across all Stages

[kmanning]

• Issue CredentialsPlugin should be applied to every Stage, not just BuildStage #192
• Previously, the CredentialsPlugin only modified the BuildStage
• Have it also inject credentials into every other Stage (RegressionStage, TerraformEnvironmentStage, TerraformValidateStage).

[codecov]

Codecov Report

Merging #212 into v5.6-dev will increase coverage by 0.45%.
The diff coverage is 37.50%.

@@             Coverage Diff              @@
##           v5.6-dev     #212      +/-   ##
============================================
+ Coverage     58.16%   58.61%   +0.45%     
============================================
  Files            32       32              
  Lines           729      737       +8     
  Branches        201      204       +3     
============================================
+ Hits            424      432       +8     
+ Misses          222      217       -5     
- Partials         83       88       +5     

Impacted Files	Coverage Δ
src/TerraformEnvironmentStage.groovy	52.30% <0.00%> (-0.82%)	⬇️
src/TerraformValidateStage.groovy	27.27% <0.00%> (+8.22%)	⬆️
src/CredentialsPlugin.groovy	62.06% <50.00%> (+5.54%)	⬆️
src/RegressionStage.groovy	40.54% <0.00%> (+2.70%)	⬆️

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