CI: add build testing

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

.github/pull_request_template.md

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+
+
+- [ ] By selecting this checkbox, I agree to license my contributions to this project under the license(s) described in the LICENSE file, and I have the right to do so or have received permission to do so by an employer or client I am producing work for whom has this right.

.github/workflows/ci.yml

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+name: CI
+on:
+  - push
+  - pull_request
+jobs:
+  x86_64-linux:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+          submodules: 'true'
+      - name: Setup Zig
+        run: |
+          sudo apt install xz-utils
+          sudo sh -c 'wget -c https://ziglang.org/builds/zig-linux-x86_64-0.11.0-dev.3380+7e0a02ee2.tar.xz -O - | tar -xJ --strip-components=1 -C /usr/local/bin'
+      - name: build
+        run: zig build