Limit the circumstances in which CI is kicked off

[SeanCurtis-TRI]

With the previous spelling, if a forked version of FCL had actions enabled, then every push to every branch would kick off a mac/ubuntu CI event. This is ridiculous and telling forkers to not allow github actions is not the solution.

So, we take two strategies to spare the FCL contributors:

1. We limit the CI to operate only on the master branch.
2. The CI tasks become no-ops unless it is run on the FCL repo. This will still cause actions to be triggered, but they will silently do nothing in zero time.

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This change is 

[SeanCurtis-TRI]

Testing strategy:

First confirm initial conditions -- there is actually a problem to be fixed:

• I've confirmed that my fork has actions enabled.
• Creating a new branch (named dummy) and pushing some changes triggers [full CI]((https://github.com/SeanCurtis-TRI/fcl/actions/runs/4483566229).
• [x] Problem confirmed!

Things to test (and how they're tested):

1. No CI gets kicked off for an arbitrary branch on the fork.
   • [x] Looking at this fork's action history shows no entry for the branch underlying this PR (PR_limit_ci).
2. No CI gets kicked off when I PR against a non-master branch on the fork.
   • [x] I created the PR https://github.com/SeanCurtis-TRI/fcl/pull/2 (this PR's commit against the dummy branch from above) which did not kick of any fork CI.
3. PR'ing against a fork's master triggers a CI, but it's a no-op.
   • [x] I created the PR https://github.com/SeanCurtis-TRI/fcl/pull/1 (this PR's commit against my fork's master) which kicked of a fork-local CI where all of the jobs were skipped.
4. However, PRs against fcl/master still get tested.
   • [x] The fact that this PR kicked off CI on the master repo shows that we haven't lost actual CI.

[SeanCurtis-TRI]

I'm simply going to merge this as it shouldn't have touched appveyor at all.