Define Go versions globally, switch to current version for most jobs

Resolves grafana#1640 (comment)

.github/workflows/all.yml

@@ -14,8 +14,10 @@ defaults:
     shell: bash
 
 env:
-  APP_NAME: "k6"
+  APP_NAME: k6
   DOCKER_IMAGE_ID: ${{ github.repository }}
+  GO_VERSION_CURRENT: 1.15.x
+  GO_VERSION_PREVIOUS: 1.14.x
 
 jobs:
   deps:
@@ -26,7 +28,7 @@ jobs:
       - name: Install Go
         uses: actions/setup-go@v2
         with:
-          go-version: 1.14.x
+          go-version: ${{ env.GO_VERSION_CURRENT }}
       - name: Check dependencies
         run: |
             go version
@@ -46,7 +48,7 @@ jobs:
       - name: Install Go
         uses: actions/setup-go@v2
         with:
-          go-version: 1.14.x
+          go-version: ${{ env.GO_VERSION_CURRENT }}
       - name: Install golangci-lint
         working-directory: /tmp
         run: go get github.com/golangci/golangci-lint/cmd/golangci-lint@$GOLANGCI_VERSION
@@ -59,7 +61,9 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [1.14.x, 1.15.x]
+        go-version:
+          - ${{ env.GO_VERSION_PREVIOUS }}
+          - ${{ env.GO_VERSION_CURRENT }}
         platform: [ubuntu-latest, windows-latest]
     runs-on: ${{ matrix.platform }}
     env:
@@ -118,7 +122,7 @@ jobs:
       - name: Install Go
         uses: actions/setup-go@v2
         with:
-          go-version: 1.14.x
+          go-version: ${{ env.GO_VERSION_CURRENT }}
       - name: Install package builders
         env:
           GO111MODULE: 'off'