Update GitHub actions to work in Taskwarrior

This moves the workspace Cargo.toml to the root of the repository, so
that the "actions-rs/cargo" action can find it.

.github/workflows/rust-tests.yml

@@ -0,0 +1,47 @@
+name: tests - rust
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    types: [opened, reopened, synchronize]
+
+jobs:
+  test:
+    strategy:
+      matrix:
+        rust:
+          # MSRV; most not be higher than the clippy rust version in checks.yml
+          - "1.47"
+          - "stable"
+        os:
+          - ubuntu-latest
+          - macOS-latest
+          - windows-latest
+
+    name: "rust ${{ matrix.rust }} on ${{ matrix.os }}"
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - uses: actions/checkout@v1
+
+      - name: Cache cargo registry
+        uses: actions/cache@v2
+        with:
+          path: ~/.cargo/registry
+          key: ${{ runner.os }}-${{ matrix.rust }}-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Cache cargo build
+        uses: actions/cache@v2
+        with:
+          path: target
+          key: ${{ runner.os }}-${{ matrix.rust }}-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
+
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: "${{ matrix.rust }}"
+          override: true
+
+      - name: test
+        run: cargo test