enhance(exec): templates and env support for exec pipeline

[ecrupper]

This is the final of three changes that will enhance the vela exec pipeline command. These changes fall under the CLI section below.

The Issue

vela exec pipeline does not work with templates. It also does not work with custom environments if the user is trying to emulate any of the default environment variables, such as VELA_BUILD_COMMIT.

Server Changes

1. Execute the local environment sourcing after the default env override. This will ensure that user-set values that would be found in the default environment will be properly interpreted by the compiler.
2. Adjustments to getTemplate that allows the compiler to read both from local file tree as well as remote using a GitHub token.

Worker Changes

1. Remove calls to merge environment with default environment found in executor/local/step.go. This call is unnecessary and will only overwrite user-supplied environment values found in the default env.

CLI Changes

1. Support for --template, --template-file, --compiler.github.token, and --compiler.github.url flags to the exec pipeline command. This will allow users the option to leverage templates in local pipeline execution, both local and remote.
2. Support for --env-file and --env-file-path to allow users to source environment variables from a file.

An aside:
Closes go-vela/community#340

[JordanSussman]

@ecrupper is it fair to say that the local-env flag will resolve go-vela/community#340?

[codecov]

Codecov Report

Merging #492 (1eaa3b6) into main (0dd737e) will decrease coverage by 0.36%.
The diff coverage is 18.64%.

@@            Coverage Diff             @@
##             main     #492      +/-   ##
==========================================
- Coverage   86.37%   86.02%   -0.36%     
==========================================
  Files         145      145              
  Lines        6310     6345      +35     
==========================================
+ Hits         5450     5458       +8     
- Misses        704      730      +26     
- Partials      156      157       +1     

Files	Coverage Δ
action/pipeline/validate.go	87.50% <100.00%> (-1.23%)	⬇️
action/pipeline/exec.go	0.00% <0.00%> (ø)
command/pipeline/exec.go	0.00% <0.00%> (ø)

[ecrupper]

@ecrupper is it fair to say that the local-env flag will resolve go-vela/community#340?

Yep! Local env is cleared prior to pipeline execution unless that flag is specified