Set the Go version of the CI workflow in one place

Updating Go means editing the go-version of six setup-go steps in
ci.yml. The workflow defines a GO_VERSION variable for this, but no
step has used it since it was added in a34bdf1a04.

Use it in all six steps. Give its value the ".x" suffix so that
setup-go keeps picking the latest patch release. This also keeps YAML
from reading a version like 1.30 as the number 1.3.

release.yml still needs its own update, because env variables don't
carry over to other workflows.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -1,7 +1,7 @@
 name: Continuous Integration
 
 env:
-  GO_VERSION: 1.25
+  GO_VERSION: 1.25.x
 
 on:
   push:
@@ -32,7 +32,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@v7
         with:
-          go-version: 1.25.x
+          go-version: ${{ env.GO_VERSION }}
       - name: Test code
         # we're passing -short so that we skip the integration tests, which will be run in parallel below
         run: |
@@ -95,7 +95,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@v7
         with:
-          go-version: 1.25.x
+          go-version: ${{ env.GO_VERSION }}
       - name: Print git version
         run: git --version
       - name: Test code
@@ -132,7 +132,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@v7
         with:
-          go-version: 1.25.x
+          go-version: ${{ env.GO_VERSION }}
       - name: Build linux binary
         run: |
           GOOS=linux go build
@@ -159,7 +159,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@v7
         with:
-          go-version: 1.25.x
+          go-version: ${{ env.GO_VERSION }}
       - name: Check Vendor Directory
         # ensure our vendor directory matches up with our go modules
         run: |
@@ -185,7 +185,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@v7
         with:
-          go-version: 1.25.x
+          go-version: ${{ env.GO_VERSION }}
       - name: Check formatting
         run: ./scripts/gofumpt-check.sh
       - name: Lint
@@ -208,7 +208,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@v7
         with:
-          go-version: 1.25.x
+          go-version: ${{ env.GO_VERSION }}
 
       - name: Download all coverage artifacts
         uses: actions/download-artifact@v8