Disable React 18 tests on PRs (vercel#70541)

New CI budget is not approved yet and the hydration tests are flaky.
There's no active work on Pages router so this is safe-ish to ignore to
unblock work on App router which doesn't run on the installed React
anyway.

.github/workflows/build_and_test.yml

@@ -202,6 +202,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
+        exclude:
+          # Excluding React 18 tests unless on `canary` branch until budget is approved.
+          - react: ${{ github.event_name == 'pull_request' && '18.3.1' }}
         group: [1/5, 2/5, 3/5, 4/5, 5/5]
         # Empty value uses default
         react: ['', '18.3.1']
@@ -219,6 +222,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
+        exclude:
+          # Excluding React 18 tests unless on `canary` branch until budget is approved.
+          - react: ${{ github.event_name == 'pull_request' && '18.3.1' }}
         group: [1/5, 2/5, 3/5, 4/5, 5/5]
         # Empty value uses default
         react: ['']
@@ -237,6 +243,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
+        exclude:
+          # Excluding React 18 tests unless on `canary` branch until budget is approved.
+          - react: ${{ github.event_name == 'pull_request' && '18.3.1' }}
         group: [1/5, 2/5, 3/5, 4/5, 5/5]
         # Empty value uses default
         # TODO: Run with React 18.
@@ -372,6 +381,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
+        exclude:
+          # Excluding React 18 tests unless on `canary` branch until budget is approved.
+          - react: ${{ github.event_name == 'pull_request' && '18.3.1' }}
         group: [1/4, 2/4, 3/4, 4/4]
         # Empty value uses default
         react: ['', '18.3.1']
@@ -389,6 +401,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
+        exclude:
+          # Excluding React 18 tests unless on `canary` branch until budget is approved.
+          - react: ${{ github.event_name == 'pull_request' && '18.3.1' }}
         group: [1/5, 2/5, 3/5, 4/5, 5/5]
         # Empty value uses default
         react: ['', '18.3.1']