Use download-rustc = "if-unchanged" in PR CI and x86_64-gnu

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.

Author: jyn514

.github/workflows/ci.yml

@@ -38,6 +38,7 @@ jobs:
   pr:
     name: "PR - ${{ matrix.name }}"
     env:
+      DOWNLOAD_RUSTC: 1
       PR_CI_JOB: 1
       CI_JOB_NAME: "${{ matrix.name }}"
       CARGO_REGISTRIES_CRATES_IO_PROTOCOL: sparse
@@ -277,8 +278,9 @@ jobs:
             os: ubuntu-20.04-8core-32gb
             env: {}
           - name: x86_64-gnu
+            env:
+              DOWNLOAD_RUSTC: 1
             os: ubuntu-20.04-4core-16gb
-            env: {}
           - name: x86_64-gnu-stable
             env:
               IMAGE: x86_64-gnu

src/ci/docker/run.sh

@@ -290,6 +290,7 @@ docker \
   --env DEPLOY \
   --env DEPLOY_ALT \
   --env CI \
+  --env DOWNLOAD_RUSTC \
   --env GITHUB_ACTIONS \
   --env GITHUB_REF \
   --env GITHUB_STEP_SUMMARY="/checkout/obj/${SUMMARY_FILE}" \

src/ci/github-actions/ci.yml

@@ -321,6 +321,7 @@ jobs:
     <<: *base-ci-job
     name: PR - ${{ matrix.name }}
     env:
+      DOWNLOAD_RUSTC: 1
       <<: [*shared-ci-variables, *public-variables]
       PR_CI_JOB: 1
     if: github.event_name == 'pull_request'
@@ -450,6 +451,8 @@ jobs:
             <<: *job-linux-8c
 
           - name: x86_64-gnu
+            env:
+              DOWNLOAD_RUSTC: 1
             <<: *job-linux-4c
 
           # This job ensures commits landing on nightly still pass the full

src/ci/run.sh

@@ -124,6 +124,10 @@ else
     RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --enable-llvm-assertions"
   fi
 
+  if [ "$DOWNLOAD_RUSTC" = 1 ]; then
+    RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.download-rustc=if-unchanged"
+  fi
+
   RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.verify-llvm-ir"
 
   # When running gcc backend tests, we need to install `libgccjit` and to not run llvm codegen