Don't use ubuntu-latest (#330)

Specify ubuntu-22.04 so that OS tooling changes can't break the workflow.

Author: cmacfarl

.github/workflows/pull-request.yaml

@@ -5,7 +5,7 @@ on:
 
 jobs:           
   build-pdf:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout Repository  
         uses: actions/checkout@v3
@@ -43,7 +43,7 @@ jobs:
           if-no-files-found: error
 
   build-html:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout Repository  
         uses: actions/checkout@v3
@@ -73,7 +73,7 @@ jobs:
           if-no-files-found: error
 
   spelling-check:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout Repository  
         uses: actions/checkout@v3
@@ -84,7 +84,7 @@ jobs:
           reporter: "github-check"
 
   link-check:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout Repository  
         uses: actions/checkout@v3
@@ -153,7 +153,7 @@ jobs:
           retention-days: 7
 
   image-check:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout Repository  
         uses: actions/checkout@v3
@@ -170,7 +170,7 @@ jobs:
         run: make -C docs/ imagecheck
 
   check-redirect:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout Repository  
         uses: actions/checkout@v3