Removes usage of set-env as it is being deprecated

set-env in Github Actions is considered a "moderate security vulnerability", see:
https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

.github/workflows/carp.yml

@@ -15,14 +15,11 @@ jobs:
 
     - uses: actions/setup-haskell@v1
 
-    - run:   echo ::set-env name=GITHUB_SHA::$GITHUB_SHA
-      shell: bash
-
     - uses: actions/cache@v1
       name: Cache ~/.stack
       with:
         path: ~/.stack
-        key: ${{ runner.os }}-stack-${{ env.GITHUB_SHA }}
+        key: ${{ runner.os }}-stack-${{ github.sha }}
         restore-keys: ${{ runner.os }}-stack-
 
     - name: Build
@@ -33,3 +30,4 @@ jobs:
 
     - name: Run Carp Tests
       run: ./run_carp_tests.sh --no_sdl
+

.github/workflows/osx.yml

@@ -13,15 +13,12 @@ jobs:
     steps:
       - name: Check out
         uses: actions/checkout@v2
-      
-      - run:   echo ::set-env name=GITHUB_SHA::$GITHUB_SHA
-        shell: bash
-      
+
       - uses: actions/cache@v1
         name: Cache ~/.stack
         with:
           path: ~/.stack
-          key: ${{ runner.os }}-stack-${{ env.GITHUB_SHA }}
+          key: ${{ runner.os }}-stack-${{ github.sha }}
           restore-keys: ${{ runner.os }}-stack-
 
       - uses: actions/setup-haskell@v1.1
@@ -40,3 +37,4 @@ jobs:
 
       - name: Run Carp Tests
         run: ./run_carp_tests.sh --no_sdl
+

.github/workflows/windows.yml

@@ -14,10 +14,6 @@ jobs:
       - name: Check out
         uses: actions/checkout@v2
 
-      - name: Save git commit SHA
-        run: echo ::set-env name=GITHUB_SHA::%GITHUB_SHA%
-        shell: cmd
-
       - name: Install Scoop
         run: |
           iwr -useb get.scoop.sh | iex
@@ -31,14 +27,14 @@ jobs:
         name: Cache stack dependencies
         with:
           path: C:\\Users\\runneradmin\\AppData\\Local\\Programs\\stack
-          key: ${{ runner.os }}-stack-deps-${{ env.GITHUB_SHA }}
+          key: ${{ runner.os }}-stack-deps-${{ github.sha }}
           restore-keys: ${{ runner.os }}-stack-deps
 
       - uses: actions/cache@v1
         name: Cache stack build
         with:
           path: C:\\Users\\runneradmin\\AppData\\Roaming\\stack\
-          key: ${{ runner.os }}-stack-build-${{ env.GITHUB_SHA }}
+          key: ${{ runner.os }}-stack-build-${{ github.sha }}
           restore-keys: ${{ runner.os }}-stack-build
 
       - name: Install Clang