github: set GOOS/GOARCH for `go list`

Currently we do not set the env variables for `go list ./...` resulting
in errors like
```
build constraints exclude all Go files in
/home/runner/work/tailscale/tailscale/chirp
```

Signed-off-by: Maisem Ali <maisem@tailscale.com>

.github/workflows/staticcheck.yml

@@ -31,16 +31,28 @@ jobs:
       run: "staticcheck -version"
 
     - name: Run staticcheck (linux/amd64)
-      run: "GOOS=linux GOARCH=amd64 staticcheck -- $(go list ./... | grep -v tempfork)"
+      env:
+        GOOS: linux
+        GOARCH: amd64
+      run: "staticcheck -- $(go list ./... | grep -v tempfork)"
 
     - name: Run staticcheck (darwin/amd64)
-      run: "GOOS=darwin GOARCH=amd64 staticcheck -- $(go list ./... | grep -v tempfork)"
+      env:
+        GOOS: darwin
+        GOARCH: amd64
+      run: "staticcheck -- $(go list ./... | grep -v tempfork)"
 
     - name: Run staticcheck (windows/amd64)
-      run: "GOOS=windows GOARCH=amd64 staticcheck -- $(go list ./... | grep -v tempfork)"
+      env:
+        GOOS: windows
+        GOARCH: amd64
+      run: "staticcheck -- $(go list ./... | grep -v tempfork)"
 
     - name: Run staticcheck (windows/386)
-      run: "GOOS=windows GOARCH=386 staticcheck -- $(go list ./... | grep -v tempfork)"
+      env:
+        GOOS: windows
+        GOARCH: "386"
+      run: "staticcheck -- $(go list ./... | grep -v tempfork)"
 
     - uses: k0kubun/action-slack@v2.0.0
       with: