Use shader's Cargo.toml for rust-gpu version/toolchain

We can still override the version and toolchain with CLI args. This
commit just changes the defaults.

I feel like there could be a lot of edge cases for this implementation.
So I consider it more of a first draft.

Notable changes:
* `Spirv` renamed to `SpirvCli`.
* New `impl SpirvSource` that does all the shader crate querying.
* CLI args change: `--spirv-builder` is now split into
  `--spirv-builder-source` and `--spirv-builder-version`.
* `--shader-crate` now lives on the `install` subcommand, which
  because it is inherited by `build` (where it was originally), means it
  doesn't make much of a difference.
* The full build test now has a teardown function making it idempotent.
* Cache folder structure now has sub folders for `spirv-builder-cli`'s
  and `rust-gpu` repos.

Things to consider:
* Can we remove `TARGET_SPECS` now, because we have a copy of the
  `rust-gpu` repo, we can get them from there instead?
* We have to copy the `shader-crate-template` outside of the workspace
  to get the tests to run. This is because recent Cargo now uses version
  4 in `Cargo.lock` and the shader crate toolchain uses an older version
  of Cargo that doesn't recognise that version. But that doesn't seem
  right. Why is Cargo looking outside of the `shader-crate-template`
  anyway??
* What's the UX expectations for changing `spirv-std` versions or
  overriding `rust-gpu` versions and toolchain? I think in most cases
  `spirv-builder-cli` rebuilds should occur because the changes will
  cause cache directory changes. But I think it'd be good to at least
  manually test this.
* Should the main build test be moved out into an integrations tests
  folder?
* I'd still like to test each `spirv-builder-cli` feature, ie:
  `spirv-builder-pre-cli` and `spirv-builder-0_10`. We're currently only
  testing `spirv-builder-pre-cli`.

.github/workflows/push.yaml

@@ -13,8 +13,12 @@ env:
 jobs:
   install-and-build-shaders:
     strategy:
+      fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os:
+          - ubuntu-latest
+          - macos-latest
+          - windows-latest
     runs-on: ${{ matrix.os }}
     defaults:
       run:
@@ -24,18 +28,15 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: moonrepo/setup-rust@v1
-      - run: rustup default stable
-      - run: rustup update
+      - name: Install Rust toolchain
+        run: |
+          rustup default stable
+          rustup update
       - run: cargo test
-      - run: cargo install --path crates/cargo-gpu
-      - run: cargo gpu install
-      - run: cargo gpu build --shader-crate crates/shader-crate-template --output-dir test-shaders
-      - run: ls -lah test-shaders
-      - run: cat test-shaders/manifest.json
 
   clippy:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - uses: moonrepo/setup-rust@v1
-      - run: cargo clippy
+      - run: cargo clippy -- --deny warnings