Enable 'rust-src' component in CI.

This fixes UI tests on Linux nightly. Without the `rust-src` component,
an error message pointing to the core libraries is not emitted as
expected. Presumably by making the sources available, the compiler has
somewhere to point to.

This commit also re-enables CI failures for the Linux debug target.

.github/workflows/ci.yml

@@ -5,7 +5,6 @@ on: [push, pull_request]
 env:
   CARGO_TERM_COLOR: always
 
-
 jobs:
   test:
     name: "${{ matrix.platform.name }} ${{ matrix.test.name }} (${{ matrix.platform.toolchain }})"
@@ -30,10 +29,6 @@ jobs:
 
     runs-on: ${{ matrix.platform.distro }}
 
-    continue-on-error: ${{ matrix.platform.name == 'Linux'
-      && matrix.platform.toolchain == 'nightly'
-      && matrix.test.name == 'Debug' }}
-
     steps:
       - name: Checkout Sources
         uses: actions/checkout@v2
@@ -69,6 +64,7 @@ jobs:
           profile: minimal
           toolchain: ${{ matrix.platform.toolchain }}
           override: true
+          components: rust-src
 
       - name: Cache Example Workspace
         if: matrix.test.name == 'Examples'