Make shellexpand fields more robust

[aaronmondal]

These turned out to be much buggier than anticipated. The new implementation behaves like the old one but no longer requires FromStr and has consistent behavior with what we'd expect from yaml-to-json conversions.

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This change is 

[aaronmondal]

+@allada +@adam-singer

Reviewable status: 0 of 2 LGTMs obtained, and 0 of 3 files reviewed, and pending CI: Analyze (javascript-typescript), Analyze (python), Bazel Dev / macos-13, Bazel Dev / macos-14, Bazel Dev / ubuntu-24.04, Cargo Dev / macos-13, Cargo Dev / ubuntu-22.04, Coverage, Installation / macos-13, Installation / macos-14, Installation / ubuntu-22.04, Local / ubuntu-22.04, NativeLink.com Cloud / Remote Cache / macos-14, NativeLink.com Cloud / Remote Cache / ubuntu-24.04, Publish image, Publish nativelink-worker-init, Publish nativelink-worker-lre-cc, Remote / large-ubuntu-22.04, Web Platform Deployment / macos-14, Web Platform Deployment / ubuntu-24.04, asan / ubuntu-22.04, docker-compose-compiles-nativelink (22.04), integration-tests (22.04), macos-13, pre-commit-checks, ubuntu-20.04 / stable, ubuntu-22.04, ubuntu-22.04 / stable, windows-2022 / stable (waiting on @adam-singer and @allada)

[adam-singer]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: 1 of 2 LGTMs obtained, and all files reviewed, and 1 discussions need to be resolved (waiting on @allada)

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nativelink-config/src/serde_utils.rs line 214 at r1 (raw file):

        fn visit_str<E: de::Error>(self, v: &str) -> Result<Self::Value, E> {
            let expanded = shellexpand::env(v).map_err(de::Error::custom)?;
            let s = expanded.as_ref().trim();

nit: let s = expanded.as_ref().trim(); -> let expanded = expanded.as_ref().trim();

[aaronmondal]

Reviewed 1 of 3 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: 1 of 2 LGTMs obtained, and all files reviewed (waiting on @allada)

[aaronmondal]

-@allada

Reviewable status: complete! 1 of 1 LGTMs obtained, and all files reviewed