Make github actions act again about rune

[Michael-F-Bryan]

Normal CI (i.e. cargo check && cargo test) takes about 10 minutes and runs on a single Linux host for every push to a PR. This can normally drop to 2-5 minutes per run if the cache is warm, but the cache is invalidated after a day because we use nightly.

These builds are by far the most important and need to be run. That said, it's in our best interest to minimise build times so everyone gets rapid feedback on their changes.

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The benchmark action takes about 30 minutes on a Linux host and is set to run every time there is a commit to master.

We don't really use the results from these runs, so I'd be happy with changing the trigger to be either weekly or just for tags.

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Notably, #76 (if merged) will introduce another set of builds because it builds the entire repo and packages it up so people aren't compiling things from source all the time.

Ideally we'd run the action every night (assuming there have been changes) and for Windows/Linux/Mac, but once you consider each build takes about 15 minutes and that Windows and Mac builds consume 2x and 10x minutes, that job alone will cost us (15 + 15*2 + 15*10) * 7 = 1365 CI minutes/week.

It could be tweaked so we run Linux nightly and only do the other builds when we do a release (i.e. a tagged commit is pushed to master). That would drop the nightly action to a more realistic 105 minutes/week.

[Michael-F-Bryan]

GitHub Actions are all set up and we get it for free now the Rune repo is public.