Unrolled build for #146563

Rollup merge of #146563 - lolbinarycat:bootstrap-ci-no-incremental, r=Kobzol

bootstrap.py: disable incremental build for bootstrap in CI

locally this seems to save a quarter of a second per build of bootstrap, presumably mainly because it avoids writing 280MB to disk.  unsure if this is worth two extra lines of python, i'll let t-bootstrap decide.

Author: rust-timer

src/bootstrap/bootstrap.py

@@ -1039,6 +1039,9 @@ def build_bootstrap_cmd(self, env):
         # See also: <https://github.com/rust-lang/rust/issues/70208>.
         if "CARGO_BUILD_TARGET" in env:
             del env["CARGO_BUILD_TARGET"]
+        # if in CI, don't use incremental build when building bootstrap.
+        if "GITHUB_ACTIONS" in env:
+            env["CARGO_INCREMENTAL"] = "0"
         env["CARGO_TARGET_DIR"] = build_dir
         env["RUSTC"] = self.rustc()
         env["LD_LIBRARY_PATH"] = (