Downsize builders for some x86_64-gnu targets

The infra-team is continuously monitoring the efficiency of the build
system in an effort to improve overall build times and resource usage.
The builders for some of the `x86_64-gnu` targets have used much less
resources than allocated in the past, so we are testing a smaller
builder size for them.

Author: jdno

.github/workflows/ci.yml

@@ -275,16 +275,16 @@ jobs:
             os: ubuntu-20.04-8core-32gb
             env: {}
           - name: x86_64-gnu
-            os: ubuntu-20.04-8core-32gb
+            os: ubuntu-20.04-4core-16gb
             env: {}
           - name: x86_64-gnu-stable
             env:
               IMAGE: x86_64-gnu
               RUST_CI_OVERRIDE_RELEASE_CHANNEL: stable
               CI_ONLY_WHEN_CHANNEL: nightly
-            os: ubuntu-20.04-8core-32gb
+            os: ubuntu-20.04-4core-16gb
           - name: x86_64-gnu-aux
-            os: ubuntu-20.04-8core-32gb
+            os: ubuntu-20.04-4core-16gb
             env: {}
           - name: x86_64-gnu-debug
             os: ubuntu-20.04-8core-32gb
@@ -309,7 +309,7 @@ jobs:
               RUST_BACKTRACE: 1
             os: ubuntu-20.04-8core-32gb
           - name: x86_64-gnu-nopt
-            os: ubuntu-20.04-8core-32gb
+            os: ubuntu-20.04-4core-16gb
             env: {}
           - name: x86_64-gnu-tools
             env:

src/ci/github-actions/ci.yml

@@ -435,7 +435,7 @@ jobs:
             <<: *job-linux-8c
 
           - name: x86_64-gnu
-            <<: *job-linux-8c
+            <<: *job-linux-4c
 
           # This job ensures commits landing on nightly still pass the full
           # test suite on the stable channel. There are some UI tests that
@@ -450,10 +450,10 @@ jobs:
               # could cause failures when `dev: 1` in `stage0.txt`, and running
               # this on stable is useless.
               CI_ONLY_WHEN_CHANNEL: nightly
-            <<: *job-linux-8c
+            <<: *job-linux-4c
 
           - name: x86_64-gnu-aux
-            <<: *job-linux-8c
+            <<: *job-linux-4c
 
           - name: x86_64-gnu-debug
             <<: *job-linux-8c
@@ -482,7 +482,7 @@ jobs:
             <<: *job-linux-8c
 
           - name: x86_64-gnu-nopt
-            <<: *job-linux-8c
+            <<: *job-linux-4c
 
           - name: x86_64-gnu-tools
             env: