dev/ci: try to make pipeline generator more predictable

Created by: bobheadxi

Attempt to address https://github.com/sourcegraph/sourcegraph/discussions/24552#discussioncomment-1287805

• Add RunType: a run can only be one at a time. With the various isXYZ this is only implied at the moment (most cases check for branch prefixes or matches that can't collide), but this makes it explicit
  • Make profiling flag a submatch on a branch so that it can be toggled on arbitrary branches
  • Removed Quick which seems unused (no recently pushed quick/ branches)
  • Attempted to document the current types (previously undocumented)
• Reduce instances of sporadic no-ops/mystery steps when adding an operation - push more conditional steps cases upfront so that it's clearer what's going on by removing Config as an argument in step constructors.
  • Looking to enforce a policy of "add variants of operations but don't conditionally add operations"
• Create type ChangedFiles []string to give change-based conditions a clearer home. Encourage its use in generating pipelines for PRs
  • Looking to create a dilineation between onlyXYZ (replace-ive) and affectsXYZ (additive)
• Random streamlining
  • Changed strangely out of place copyEnv which copies from os.Getenv even though we have another thing (commonEnv) which also copies from os.Getenv to copy from the latter

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Some other things I'm contemplating (maybe another PR):

• sg pipeline plan
• debug output
• tests???