Add var step to workflows

[lkysow]

Add new var step for setting environment variable in workflows.

Squashes and refactors #656, fixes #370

[codecov]

Codecov Report

Merging #751 into master will increase coverage by 0.4%.
The diff coverage is 97.93%.

@@            Coverage Diff            @@
##           master     #751     +/-   ##
=========================================
+ Coverage   72.25%   72.65%   +0.4%     
=========================================
  Files          62       63      +1     
  Lines        4728     4794     +66     
=========================================
+ Hits         3416     3483     +67     
+ Misses       1057     1056      -1     
  Partials      255      255

Impacted Files	Coverage Δ
server/events/yaml/valid/repo_cfg.go	19.04% <ø> (ø)	⬆️
server/events/runtime/runtime.go	78.94% <ø> (ø)	⬆️
server/events/runtime/init_step_runner.go	83.33% <100%> (ø)	⬆️
server/events/runtime/plan_step_runner.go	87.2% <100%> (ø)	⬆️
server/server.go	64.35% <100%> (+0.35%)	⬆️
server/events/runtime/run_step_runner.go	90.47% <100%> (+0.47%)	⬆️
server/events/runtime/env_step_runner.go	100% <100%> (ø)
server/events/runtime/apply_step_runner.go	85.98% <100%> (ø)	⬆️
server/events/project_command_runner.go	74.1% <100%> (+1.2%)	⬆️
server/events/yaml/raw/step.go	96.12% <100%> (+5.42%)	⬆️
... and 3 more

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