vergen: idempotent builds from CI only, I guess?

[iximeow]

see the diff; it turns out that setting .idempotent() omits setting rerun-if-changed=.git/HEAD. then if you:

• building Propolis at A (fresh or otherwise with no cached builds),
• check out a different commit B,
• build again without cargo clean or causing propolis to rebuild

you end up with propolis-server and propolis-standalone binaries reporting they were built at commit A while containing code that quite definitely is from B.

this should fix getting accurate commit data when you're making development builds from moving around in git, but keep things more determinsitic in CI.

[iximeow]

there's another bug that I'm going to push another commit here for in a second: if your working tree was clean and became dirty without touching propolis-lib (or was dirty and became clean in a similar way), the version string will become incorrect with respect to the state of the tree. an easy way to see this is add eprintln("hi :)"); to main() anywhere and rebuild; -V will say the build is from the corresponding commit when it just printed hi :) that we certainly won't ever have in-tree!

the fix for that will be moving the vergen instructions around a bit so that git-tree-is-dirty comes from the bin crates where the rest comes from propolis-lib. since they're all built out of the same directory, at least centralizing the git commit state works reasonably..

[hawkw]

this all makes sense to me; the rerun-if-changed sucks but there is really no good solution for it, so I think this is the least bad solution!

[hawkw]

every (e.g. both)

okay, this bit got me, good show!