fix: read CARGO_MANIFEST_DIR at runtime

[tomgr]

Using env! inside the build.rs script makes it difficult to build this package with other build tools like bazel which sandbox execution, since the source directory might be at different paths between when the build.rs file is compiled, and when the resulting build script is executed.

Cargo recommends that CARGO_MANIFEST_DIR is read at runtime instead: https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-build-scripts

The above links also includes the promise that Cargo will set the cwd to the crate's directory. This patch uses this to find the descriptors.bin file.

[echochamber]

Any chance this can get merged. I just tested the CI builds on my fork and the tests pass, The CI actions that are failing are :

• Lint - only failing because of using a deprecated methods from the chrono crate,
• Audit - Failing because the rustc version on the image is too old for toml_edit, one of the deps for cargo audit.

I think neither of the above need to block this pull request since they are unrelated to this issue and can be fixed in their own separate PRs