CI: split x86_64-mingw job

src/bootstrap/mk/Makefile.in

@@ -108,11 +108,14 @@ ci-msvc: ci-msvc-py ci-msvc-ps1
 
 ## MingW native builders
 
+# Set of tests that should represent half of the time of the test suite.
+# Used to split tests across multiple CI runners.
+
 # test both x and bootstrap entrypoints
 ci-mingw-x:
-	$(Q)$(CFG_SRC_DIR)/x test --stage 2 tidy
+	$(Q)$(BOOTSTRAP) test --stage 2 tests/
 ci-mingw-bootstrap:
-	$(Q)$(BOOTSTRAP) test --stage 2 --skip tidy
+	$(Q)$(BOOTSTRAP) test --stage 2 --skip tests/
 ci-mingw: ci-mingw-x ci-mingw-bootstrap
 
 .PHONY: dist

src/ci/github-actions/jobs.yml

@@ -431,14 +431,24 @@ auto:
       NO_DOWNLOAD_CI_LLVM: 1
     <<: *job-windows-8c
 
-  - image: x86_64-mingw
+  # x86_64-mingw is split into two jobs to run tests in parallel.
+  - image: x86_64-mingw-1
     env:
-      SCRIPT: make ci-mingw
+      SCRIPT: make ci-mingw-x
       RUST_CONFIGURE_ARGS: --build=x86_64-pc-windows-gnu
       # We are intentionally allowing an old toolchain on this builder (and that's
       # incompatible with LLVM downloads today).
       NO_DOWNLOAD_CI_LLVM: 1
-    <<: *job-windows-8c
+    <<: *job-windows
+
+  - image: x86_64-mingw-2
+    env:
+      SCRIPT: make ci-mingw-bootstrap
+      RUST_CONFIGURE_ARGS: --build=x86_64-pc-windows-gnu
+      # We are intentionally allowing an old toolchain on this builder (and that's
+      # incompatible with LLVM downloads today).
+      NO_DOWNLOAD_CI_LLVM: 1
+    <<: *job-windows
 
   - image: dist-x86_64-msvc
     env:

src/ci/shared.sh

@@ -53,7 +53,8 @@ function isLinux {
 }
 
 function isKnownToBeMingwBuild {
-    isGitHubActions && [[ "${CI_JOB_NAME}" == *mingw ]]
+    # CI_JOB_NAME must end with "mingw" and optionally `-N` to be considered a MinGW build.
+    isGitHubActions && [[ "${CI_JOB_NAME}" =~ mingw(-[0-9]+)?$ ]]
 }
 
 function isCiBranch {