Use download-rustc = "if-unchanged" in PR CI

This has two main benefits:

1. It tests that download-rustc doesn't regress. This doesn't reduce our test coverage, since we
   still never use `download-rustc` in a full bors merge, but it should make it a lot less likely that
   this breaks by accident.

2. It greatly speeds up CI when compiler/ and library/ haven't been modified. Once the changes in
   rust-lang/compiler-team#619 land, this will also be faster for changes to
   library/, and only changes to compiler/ will have to rebuild.

.github/workflows/ci.yml

@@ -36,6 +36,7 @@ jobs:
       actions: write
     name: "PR - ${{ matrix.name }}"
     env:
+      DOWNLOAD_RUSTC: 1
       CI_JOB_NAME: "${{ matrix.name }}"
       CARGO_REGISTRIES_CRATES_IO_PROTOCOL: sparse
       SCCACHE_BUCKET: rust-lang-ci-sccache2

src/ci/docker/run.sh

@@ -254,6 +254,7 @@ docker \
   --env DEPLOY \
   --env DEPLOY_ALT \
   --env CI \
+  --env DOWNLOAD_RUSTC \
   --env TF_BUILD \
   --env BUILD_SOURCEBRANCHNAME \
   --env GITHUB_ACTIONS \

src/ci/github-actions/ci.yml

@@ -296,6 +296,7 @@ jobs:
     <<: *base-ci-job
     name: PR - ${{ matrix.name }}
     env:
+      DOWNLOAD_RUSTC: 1
       <<: [*shared-ci-variables, *public-variables]
     if: github.event_name == 'pull_request'
     continue-on-error: ${{ matrix.name == 'mingw-check-tidy' }}

src/ci/run.sh

@@ -83,6 +83,10 @@ if [ "$DIST_SRC" = "" ]; then
   RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --disable-dist-src"
 fi
 
+if [ "$DOWNLOAD_RUSTC" = 1 ]; then
+  RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.download-rustc=if-unchanged"
+fi
+
 # Always set the release channel for bootstrap; this is normally not important (i.e., only dist
 # builds would seem to matter) but in practice bootstrap wants to know whether we're targeting
 # master, beta, or stable with a build to determine whether to run some checks (notably toolstate).