Extract export validation into separate CI job

It makes sense to have it in a separate job, so we get better feedback
on what specifically failed, if the CI build does fail because of it.

Also, I'm having trouble getting the actual 3MF validation to work on
macOS, due to linking issues. I figure that running it on Linux is good
enough for now.

.github/workflows/ci.yml

@@ -62,6 +62,28 @@ jobs:
           token: ${{ secrets.GITHUB_TOKEN }}
           args: --all-features
 
+  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:
@@ -88,7 +110,3 @@ jobs:
       - uses: actions-rs/cargo@4ff6ec2846f6e7217c1a9b0b503506665f134c4b
         with:
           command: test
-      - uses: actions-rs/cargo@4ff6ec2846f6e7217c1a9b0b503506665f134c4b
-        with:
-          command: run
-          args: --package export-validator