.github/workflows: establish basic build and test actions jobs

Upstream doesn't use GitHub actions for CI as GitHub is simply a mirror.
Our workflows involve GitHub, so establish some basic CI jobs.

Updates tailscale/corp#28877

Signed-off-by: Jordan Whited <jordan@tailscale.com>

.github/workflows/test.yml

@@ -0,0 +1,68 @@
+name: CI
+
+on:
+  push:
+    branches: ["tailscale"]
+  pull_request:
+    branches: ["tailscale"]
+
+jobs:
+  build:
+    runs-on: ubuntu-22.04
+    strategy:
+      matrix:
+        include:
+          - goos: linux
+            goarch: amd64
+          - goos: linux
+            goarch: arm64
+          - goos: linux
+            goarch: "386"
+          - goos: linux
+            goarch: loong64
+          - goos: linux
+            goarch: arm
+            goarm: "5"
+          - goos: linux
+            goarch: arm
+            goarm: "7"
+          # macOS
+          - goos: darwin
+            goarch: amd64
+          - goos: darwin
+            goarch: arm64
+          # Windows
+          - goos: windows
+            goarch: amd64
+          - goos: windows
+            goarch: arm64
+          # BSDs
+          - goos: freebsd
+            goarch: amd64
+          - goos: openbsd
+            goarch: amd64
+    steps:
+    - name: checkout
+      uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+    - name: setup go
+      uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5 # v5.5.0
+      with:
+        go-version-file: go.mod
+    - name: build
+      run: go build ./...
+      env:
+        GOOS: ${{ matrix.goos }}
+        GOARCH: ${{ matrix.goarch }}
+        CGO_ENABLED: "0"
+
+  test:
+    runs-on: ubuntu-22.04
+    steps:
+    - name: checkout
+      uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+    - name: setup go
+      uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5 # v5.5.0
+      with:
+        go-version-file: go.mod
+    - name: test
+      run: go test -race -v ./...