Replace current_dir().unwrap() in init() with env::var_os("CARGO_MANIFEST_DIR")

[leighmcculloch]

What

Replace current_dir().unwrap() in init() with env::var_os("CARGO_MANIFEST_DIR"), and document why this is the correct source of the crate path in a build script.

Why

current_dir() happens to equal the manifest directory in the common case, but package.build in Cargo.toml can point to a build script located outside the package source tree. CARGO_MANIFEST_DIR is the env var Cargo explicitly sets to the package's manifest directory when executing the build script, so it's the correct source for the crate path regardless of where the build script lives.

Using expect instead of unwrap also surfaces a clear message at the panic site. Under normal operations there is no reason for CARGO_MANIFEST_DIR to be unset — Cargo always sets it for build scripts. If it is unset, the build system is already broken in other, larger, disastrous ways, and graceful handling here would not meaningfully recover the build.

Some discussion:

• https://stellarfoundation.slack.com/archives/C09UBQTVBV2/p1779770578107319

[Copilot]

Pull request overview

This PR improves the clarity of failures in crate_git_revision::init() by replacing a bare unwrap() on std::env::current_dir() with an expect(...) message, and adds documentation explaining why relying on the current working directory is correct when invoked from a Cargo build script.

Changes:

• Replace current_dir().unwrap() with current_dir().expect("getting current directory") in init().
• Add an in-function comment (with a Cargo reference link) describing Cargo’s working-directory guarantee for build scripts.
• Update the public rustdoc for init() to describe the working-directory requirement (but it currently describes it imprecisely).

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