Remove some job matrices

Co-authored-by: Curtis Vogt <curtis.vogt@gmail.com>

.github/workflows/pr_checks.yml

@@ -79,12 +79,7 @@ jobs:
       - run: julia ./ci/check_uncommitted_changes.jl
   build:
     timeout-minutes: 30
-    runs-on: ${{ matrix.os }}
-    strategy:
-      fail-fast: false
-      matrix:
-        os:
-          - ubuntu-latest
+    runs-on: ubuntu-latest
     steps:
       ### Check out the repo:
       - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332
@@ -128,12 +123,7 @@ jobs:
           make cleanall
   npm-run-test:
     timeout-minutes: 30
-    runs-on: ${{ matrix.os }}
-    strategy:
-      fail-fast: false
-      matrix:
-        os:
-          - ubuntu-latest
+    runs-on: ubuntu-latest
     steps:
       ### Check out the repo:
       - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332
@@ -168,12 +158,7 @@ jobs:
       - run: make test
   make-targets: # This is a job to make sure that none of the `make` targets bitrot
     timeout-minutes: 30
-    runs-on: ${{ matrix.os }}
-    strategy:
-      fail-fast: false
-      matrix:
-        os:
-          - ubuntu-latest
+    runs-on: ubuntu-latest
     steps:
       ### Check out the repo:
       - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332