diff --git a/.github/workflows/oldest.yml b/.github/workflows/oldest.yml
index c73311ae0..d3b1c0c42 100644
--- a/.github/workflows/oldest.yml
+++ b/.github/workflows/oldest.yml
@@ -42,58 +42,58 @@ jobs:
         # XXX if stack.yaml is a symlink, this fails with
         #  Error: The template is not valid. .github/workflows/push.yml (Line: 103, Col: 14): hashFiles('**.yaml') failed.
         #  Fail to hash files under directory '/home/runner/work/hledger/hledger'
-        key: ${{ runner.os }}-stack-global-$ghc-${{ hashFiles('**.yaml') }}
+        key: ${{ runner.os }}-stack-global-${{ env.ghc }}-${{ hashFiles('**.yaml') }}
         restore-keys: |
-             ${{ runner.os }}-stack-global-$ghc
+             ${{ runner.os }}-stack-global-${{ env.ghc }}
 
     - name: Cache - stack-installed programs in ~/.local/bin
       id:   stack-programs
       uses: actions/cache@v4
       with:
         path: ~/.local/bin
-        key: ${{ runner.os }}-stack-programs-$ghc-${{ hashFiles('**.yaml') }}
+        key: ${{ runner.os }}-stack-programs-${{ env.ghc }}-${{ hashFiles('**.yaml') }}
         restore-keys: |
-             ${{ runner.os }}-stack-programs-$ghc
+             ${{ runner.os }}-stack-programs-${{ env.ghc }}
 
     - name: Cache - .stack-work
       uses: actions/cache@v4
       with:
         path: .stack-work
-        key: ${{ runner.os }}-stack-work-$ghc-${{ hashFiles('**.yaml') }}
+        key: ${{ runner.os }}-stack-work-${{ env.ghc }}-${{ hashFiles('**.yaml') }}
         restore-keys: |
-             ${{ runner.os }}-stack-work-$ghc
+             ${{ runner.os }}-stack-work-${{ env.ghc }}
 
     - name: Cache - hledger-lib/.stack-work
       uses: actions/cache@v4
       with:
         path: hledger-lib/.stack-work
-        key: ${{ runner.os }}-hledger-lib-stack-work-$ghc-${{ hashFiles('hledger-lib/package.yaml') }}
+        key: ${{ runner.os }}-hledger-lib-stack-work-${{ env.ghc }}-${{ hashFiles('hledger-lib/package.yaml') }}
         restore-keys: |
-             ${{ runner.os }}-hledger-lib-stack-work-$ghc
+             ${{ runner.os }}-hledger-lib-stack-work-${{ env.ghc }}
 
     - name: Cache - hledger/.stack-work
       uses: actions/cache@v4
       with:
         path: hledger/.stack-work
-        key: ${{ runner.os }}-hledger-stack-work-$ghc-${{ hashFiles('hledger/package.yaml') }}
+        key: ${{ runner.os }}-hledger-stack-work-${{ env.ghc }}-${{ hashFiles('hledger/package.yaml') }}
         restore-keys: |
-             ${{ runner.os }}-hledger-stack-work-$ghc
+             ${{ runner.os }}-hledger-stack-work-${{ env.ghc }}
 
     - name: Cache - hledger-ui/.stack-work
       uses: actions/cache@v4
       with:
         path: hledger-ui/.stack-work
-        key: ${{ runner.os }}-hledger-ui-stack-work-$ghc-${{ hashFiles('hledger-ui/package.yaml') }}
+        key: ${{ runner.os }}-hledger-ui-stack-work-${{ env.ghc }}-${{ hashFiles('hledger-ui/package.yaml') }}
         restore-keys: |
-             ${{ runner.os }}-hledger-ui-stack-work-$ghc
+             ${{ runner.os }}-hledger-ui-stack-work-${{ env.ghc }}
 
     - name: Cache - hledger-web/.stack-work
       uses: actions/cache@v4
       with:
         path: hledger-web/.stack-work
-        key: ${{ runner.os }}-hledger-web-stack-work-$ghc-${{ hashFiles('hledger-web/package.yaml') }}
+        key: ${{ runner.os }}-hledger-web-stack-work-${{ env.ghc }}-${{ hashFiles('hledger-web/package.yaml') }}
         restore-keys: |
-             ${{ runner.os }}-hledger-web-stack-work-$ghc
+             ${{ runner.os }}-hledger-web-stack-work-${{ env.ghc }}
 
 
     # ACTIONS