Integrate export validation into "Test" job

This has two advantages:

- It runs export validation on all supported platforms, which can catch
  platform-specific issues.
- It saves one GitHub Actions job.

.github/workflows/ci.yml

@@ -92,28 +92,6 @@ jobs:
         env:
           RUSTDOCFLAGS: -D warnings
 
-  export:
-    name: Validate 3MF Export
-    runs-on: ubuntu-latest
-    steps:
-      - name: Check out repository
-        uses: actions/checkout@v3
-      - name: Set up toolchain
-        uses: oxidecomputer/actions-rs_toolchain@oxide/master
-        # see https://github.com/actions-rs/toolchain/pull/209
-        # uses: actions-rs/toolchain@v1
-        with:
-          override: true
-          profile: minimal
-          target: ${{ matrix.target }}
-      - name: Set up Rust cache
-        uses: Swatinem/rust-cache@1232abb8968faf344409165de17cbf9e7f340fd8
-      - name: Run `export-validator`
-        uses: actions-rs/cargo@4ff6ec2846f6e7217c1a9b0b503506665f134c4b
-        with:
-          command: run
-          args: --package export-validator
-
   test:
     name: Test
     strategy:
@@ -149,3 +127,8 @@ jobs:
         with:
           command: test
           args: --all-features
+      - name: Run `export-validator`
+        uses: actions-rs/cargo@4ff6ec2846f6e7217c1a9b0b503506665f134c4b
+        with:
+          command: run
+          args: --package export-validator