collector: passthrough env vars

This commit modifies the collector to stop clearing the environment when
invoking the fake rustc.

This is helpful for users of Nix development shells where environment
variables are required to inform tools of the location of C
dependencies, and for setting RUSTFLAGS to collect data from the same
build with different flags.

However, this change can result in some environment variables affecting
the data collection where that isn't intended.

Signed-off-by: David Wood <[email protected]>

Author: davidtwco

collector/src/bin/rustc-perf-collector/execute.rs

@@ -232,17 +232,6 @@ impl<'a> CargoProcess<'a> {
         cmd
             // Not all cargo invocations (e.g. `cargo clean`) need all of these
             // env vars set, but it doesn't hurt to have them.
-            .env_clear()
-            // SHELL is needed for some benchmarks' build scripts.
-            .env("SHELL", env::var_os("SHELL").unwrap_or_default())
-            // PATH is needed to find things like linkers used by rustc/Cargo.
-            .env("PATH", env::var_os("PATH").unwrap_or_default())
-            // HOME is needed for cargo to find its home directory.
-            .env("HOME", env::var_os("HOME").unwrap_or_default())
-            .env(
-                "RUSTC_THREAD_COUNT",
-                env::var_os("RUSTC_THREAD_COUNT").unwrap_or_default(),
-            )
             .env("RUSTC", &*FAKE_RUSTC)
             .env("RUSTC_REAL", &self.compiler.rustc)
             .env(