Conserve CI minutes via cache, skip macOS+Windows MSRV builds (#827)

Windows and macOS runners consume GitHub Actions minutes at [2x and 10x
the rate of Linux runners,
respectively](https://docs.github.com/en/billing/managing-billing-for-github-actions/about-billing-for-github-actions#minute-multipliers).
This is a bit concerning now that there are two Windows jobs and two
macOS jobs, introduced in #825.

This PR aims to reduce the cost by:
* Adding [rust-cache](https://github.com/Swatinem/rust-cache/) to reduce
the amount of time spent. I'm aware there were some concerns raised
about CI caches in general in #496 - are they still a blocker?
* Removing the unnecessary Windows and macOS MSRV build jobs. If an MSRV
build fails on one platform due to usage of new language features, then
it will fail on all of them.

@Kampfkarren may have to change this repository's required status checks
before this PR can be merged

.github/workflows/ci.yml

@@ -16,7 +16,6 @@ jobs:
 
     strategy:
       matrix:
-        rust_version: [stable, 1.70.0]
         os: [ubuntu-latest, windows-latest, macos-latest]
 
     steps:
@@ -27,10 +26,13 @@ jobs:
     - name: Install Rust
       uses: actions-rs/toolchain@v1
       with:
-        toolchain: ${{ matrix.rust_version }}
+        toolchain: stable
         override: true
         profile: minimal
 
+    - name: Rust cache
+      uses: Swatinem/rust-cache@v2
+
     - name: Setup Aftman
       uses: ok-nick/setup-aftman@v0.3.0
       with:
@@ -42,6 +44,33 @@ jobs:
     - name: Test
       run: cargo test --locked --verbose
 
+  msrv:
+    name: Check MSRV
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v3
+      with:
+        submodules: true
+
+    - name: Install Rust
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: 1.70.0
+        override: true
+        profile: minimal
+
+    - name: Rust cache
+      uses: Swatinem/rust-cache@v2
+
+    - name: Setup Aftman
+      uses: ok-nick/setup-aftman@v0.3.0
+      with:
+        version: 'v0.2.7'
+
+    - name: Build
+      run: cargo build --locked --verbose
+
   lint:
     name: Rustfmt, Clippy, & Stylua
     runs-on: ubuntu-latest
@@ -58,6 +87,9 @@ jobs:
         override: true
         components: rustfmt, clippy
 
+    - name: Rust cache
+      uses: Swatinem/rust-cache@v2
+
     - name: Setup Aftman
       uses: ok-nick/setup-aftman@v0.3.0
       with: