Description of changes:

We want to rebuild the model package when changing variants, but before
this change, we rebuild every time.  This happens because cargo detects
that the timestamps of the variant-sensitive links we create (and that
we tell it to watch) don't match its output file.  We don't control
creation of the output file, so they'll never match.  Each rebuild
changes the links again, causing another rebuild.

This change sets the mtime of the links to epoch, effectively telling
cargo that they haven't changed.  We still rebuild if the variant
changes, or if someone changes the links behind our back, which would
result in a different mtime.

If you're curious, you can figure this kind of thing out like this:

$ VARIANT=aws-k8s-1.19 \
   CARGO_LOG=cargo::core::compiler::fingerprint=info \
   cargo build
...
INFO  cargo::core::compiler::fingerprint] stale: changed "/home/tjk/work/bottlerocket/sources/models/src/variant/current"
...

Testing done:

• Normal cargo build works, another normal cargo build doesn't rebuild, yay.
• Changing VARIANT causes a rebuild; another build with that VARIANT doesn't rebuild, yay.
• Probably the biggest quality of life improvement for me: cargo running apiclient multiple times doesn't rebuild every time.
• Created a k8s AMI, tested fine; created an ECS AMI, tested fine; no model caching issues.
• Confirmed that I could change the link behind our back, set its mtime to epoch, and work around the check if I really wanted to, which I hope no one does.

Performance effects:

• Obviously, local rebuilds of API-related crates are infinitely faster because they don't rebuild unnecessarily.
• Doing touch os.spec and rebuilding the os package shows that the initial build takes ~3.5 minutes, as expected, but repeat attempts only take ~52 seconds since cargo knows all the Rust code is already built and it can skip right to license scanning and whatnot.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.