chore(ci): replace mbta/actions/reshim-asdf

[rwaskiewicz]

Asana task: N/A

Description
Replace mbta/actions/reshim-asdf with bash equivalent commands. In
the v2.20 release, commit mbta/actions@3579cb7
marked the reshim-asdf action as deprecated. A recent Dependabot update
attempted to bump this action's version to v2.21 - while the update
would succeed otherwise, replace the action now while it's on top of
mind.

(Note: Since we're using tags instead of SHAs for this action, we're technically already using v2.20 (v2 still points at one minor behind as of this writing) in its deprecated state)

The existing job structure of the CI workflow has been maintained as-is.
In the future, there is room to refactor this to either a composite
action or series of parallel steps, which would eliminate the need to
pass the ASDF_DIR and ASDF_DATA_DIR values between jobs. However,
since this is an unplanned task created to avoid a deprecated Action,
leave that for another day.

asdf-vm/actions/setup was not used here. While the action can achieve
the same effect as the new asdf_env_setup step, it did not seem worth
it to have to manage an additional dependency for setting up something
on our PATH. In the future, when Dependabot PRs are grouped and/or the
CI workflow is refactored, this can certainly be re-explored (i.e. this
is a judgment call more than anything).

• For features with a design/UX component, deployed branch to dev-green and let product know it's ready for review.

[digitalcora]

asdf-vm/actions/setup was not used here. While the action can achieve the same effect as the new asdf_env_setup step, it did not seem worth it to have to manage an additional dependency for setting up something on our PATH. In the future, when Dependabot PRs are grouped and/or the CI workflow is refactored, this can certainly be re-explored (i.e. this is a judgment call more than anything).

Mm, I don't love this, for a few reasons:

• Introduces more complexity/hand-rolled code for a situation where a supported "library" already exists
• We are already using setup in Screens, so this is introducing a third way of doing ASDF setup among our projects, instead of using one of the two existing ways
• "the future" could very easily be now — the change to group Actions updates for Dependabot would be tiny, and in that case the overhead of keeping the additional dependency up-to-date becomes also tiny

Does this sway your judgment? 😄