fix(multivers): do not propagate CARGO_UNSTABLE_BUILD_STD to the runn…

…er build

If a user adds `-Zbuild-std` to the command line to rebuild the std it is only propagated to the build of each version, but not the runner.

These flags, however, can also be configured with [environment variables](rust-lang/cargo#8393).
So if a user uses the `CARGO_UNSTABLE_BUILD_STD` environment variable to rebuild the std of the crate, it is propagated to the build of the runner. Since the runner adds `panic=abort`, if the user does not add `CARGO_UNSTABLE_BUILD_STD=std,panic_abort`, there are duplicate lang item errors.
These errors might be confusing since the build of the runner is not necessarily known, and its profile even less.

See #7

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/).
 
 ## [Unreleased]
 
+### Fixed
+
+- Do not propagate the `CARGO_UNSTABLE_BUILD_STD` environment variable to the build of the runner (#7).
+
 [Unreleased]: https://github.com/ronnychevalier/cargo-multivers/compare/v0.7.0...HEAD
 
 ## [0.7.0] - 18-12-2023

src/runner.rs

@@ -78,11 +78,16 @@ impl RunnerBuilder {
         builds_path: &Path,
         original_filename: &OsStr,
     ) -> anyhow::Result<PathBuf> {
+        // We do not propagate `CARGO_UNSTABLE_BUILD_STD` since if `panic_abort` is not
+        // specified, the build of the runner will fail (since its profile specifies `panic=abort`).
+        // A proper fix could be to clear the whole environment before spawning this `cargo build`,
+        // but until `CargoBuild` exposes the `Command` or this function, we can only do this.
         let cargo = CargoBuild::new()
             .release()
             .target(target)
             .target_dir(&self.output_directory)
             .manifest_path(&self.manifest_path)
+            .env_remove("CARGO_UNSTABLE_BUILD_STD")
             .env("MULTIVERS_BUILDS_DESCRIPTION_PATH", builds_path);
 
         let cargo = cargo