ci: use go-version-file: go.mod instead of pinning GO_VERSION

Removes the env-var coupling so the workflow's Go version follows
whatever go.mod declares. Addresses review feedback on crossplane#664.

Signed-off-by: JP Schoombee <jpschoombee@gmail.com>

Author: jschoombee

.github/workflows/ci.yml

@@ -14,7 +14,6 @@ on:
 
 env:
   # Common versions
-  GO_VERSION: "1.25.10"
   GOLANGCI_VERSION: "v2.10.1"
   DOCKER_BUILDX_VERSION: "v0.28.0"
 
@@ -58,7 +57,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
         with:
-          go-version: ${{ env.GO_VERSION }}
+          go-version-file: go.mod
 
       - name: Find the Go Build Cache
         id: go
@@ -112,7 +111,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
         with:
-          go-version: ${{ env.GO_VERSION }}
+          go-version-file: go.mod
 
       - name: Find the Go Build Cache
         id: go-cache-paths
@@ -162,7 +161,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
         with:
-          go-version: ${{ env.GO_VERSION }}
+          go-version-file: go.mod
 
       - name: Find the Go Build Cache
         id: go-cache-paths