buildsys: add CI/CD hack, fix mtime for external files

[bcressey]

Issue number:
N/A

Description of changes:
Two buildsys changes, one potentially controversial, one hopefully not.

The first is the BUILDSYS_CICD_HACK variable - naming suggestions wanted! - to allow cargo to recreate crates without actually running expensive package builds. It's a hack because it assumes the build directory is fully populated; if it's not, then bad things will happen the next time a package is rebuilt.

The second fixes an edge case related to external files or bundled Go modules; these can end up being tracked by cargo for changes, so we need to ensure that their modification time is no later than the start of the build. Cargo.toml is already tracked for changes and is a useful sentinel for this purpose.

Testing done:
Tested in bottlerocket-os/bottlerocket-core-kit#67. With both changes, I was able to reuse cached package builds, touch the files modified by a pull request, and observe that only the affected packages were rebuilt.

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.

[cbgbt]

🚀

[bcressey]

First force push handles the rebase.

Second force push adds some documentation, and adjusts the argument parsing to take a bool instead of an string value that's ignored.

Usage: buildsys build-package ...
...
      --cicd-hack
          cicd_hack is used to suppress builds from running after all the cargo-related metadata is emitted. This allows cargo to create a fresh crate, and assumes that the corresponding build artifacts are already present. It is intended for use in a CI/CD scenario where some other process populates the build directory from a cache. Other uses may lead to unexpected build failures that are difficult to troubleshoot [env: BUILDSYS_CICD_HACK=]

Testing:

❯ cargo run --bin buildsys -- build-package ...
[tools/buildsys/src/main.rs:131:5] args = BuildPackageArgs {
...
    common: Common {
        ...,
        cicd_hack: false,
    },
}

❯ cargo run --bin buildsys -- build-package --cicd-hack ...
[tools/buildsys/src/main.rs:131:5] args = BuildPackageArgs {
...
    common: Common {
        ...,
        cicd_hack: true,
    },
}

❯ BUILDSYS_CICD_HACK=true cargo run --bin buildsys -- build-package ...
[tools/buildsys/src/main.rs:131:5] args = BuildPackageArgs {
...
    common: Common {
        ...,
        cicd_hack: true,
    },
}

❯ BUILDSYS_CICD_HACK=false cargo run --bin buildsys -- build-package ...
[tools/buildsys/src/main.rs:131:5] args = BuildPackageArgs {
...
    common: Common {
        ...,
        cicd_hack: false,
    },
}