ci: use stable toolchain for compiling xtask

The nightly rust compiler is currently unable to compile the version of
fatfs we are using in xtask. Rust bug:
rust-lang/rust#95538. But we don't need to use
nightly to compile xtask. In the CI, install both stable and nightly
toolchains. The stable toolchain will be used by default, and xtask
already knows to insert `+nightly` when building uefi-rs, so there's no
need to set `override: true` for the nightly toolchain anymore.

Fixes #397

Author: nicholasbishop

Diff for: .github/workflows/rust.yml

@@ -27,11 +27,15 @@ jobs:
         cp /usr/share/AAVMF/AAVMF_CODE.fd uefi-test-runner/QEMU_EFI-pflash.raw
         cp /usr/share/AAVMF/AAVMF_VARS.fd uefi-test-runner/vars-template-pflash.raw
 
+    - name: Install stable
+      uses: actions-rs/toolchain@v1
+      with:
+          toolchain: stable
+
     - name: Install latest nightly
       uses: actions-rs/toolchain@v1
       with:
           toolchain: nightly
-          override: true
           components: rust-src
       # TODO: cache Rust binaries
 
@@ -54,11 +58,15 @@ jobs:
         sudo apt-get update
         sudo apt-get install qemu-system-x86 ovmf -y
 
+    - name: Install stable
+      uses: actions-rs/toolchain@v1
+      with:
+          toolchain: stable
+
     - name: Install latest nightly
       uses: actions-rs/toolchain@v1
       with:
           toolchain: nightly
-          override: true
           components: rust-src
       # TODO: cache Rust binaries
 
@@ -97,11 +105,15 @@ jobs:
         curl -o uefi-test-runner/OVMF32_CODE.fd https://raw.githubusercontent.com/retrage/edk2-nightly/${EDK2_NIGHTLY_COMMIT}/bin/RELEASEIa32_OVMF_CODE.fd
         curl -o uefi-test-runner/OVMF32_VARS.fd https://raw.githubusercontent.com/retrage/edk2-nightly/${EDK2_NIGHTLY_COMMIT}/bin/RELEASEIa32_OVMF_VARS.fd
 
+    - name: Install stable
+      uses: actions-rs/toolchain@v1
+      with:
+          toolchain: stable
+
     - name: Install latest nightly
       uses: actions-rs/toolchain@v1
       with:
           toolchain: nightly
-          override: true
           components: rust-src
       # TODO: cache Rust binaries
 
@@ -119,16 +131,20 @@ jobs:
       - name: Checkout sources
         uses: actions/checkout@v2
 
+      - name: Install stable
+        uses: actions-rs/toolchain@v1
+        with:
+            toolchain: stable
+
       - name: Install latest nightly
         uses: actions-rs/toolchain@v1
         with:
             profile: minimal
             toolchain: nightly
             components: rust-src
-            override: true
 
       - name: Run cargo test
-        run: cargo xtask test
+        run: cargo +stable xtask test
 
   lints:
     name: Lints
@@ -137,13 +153,17 @@ jobs:
       - name: Checkout sources
         uses: actions/checkout@v2
 
+      - name: Install stable
+        uses: actions-rs/toolchain@v1
+        with:
+            toolchain: stable
+
       - name: Install latest nightly
         uses: actions-rs/toolchain@v1
         with:
             profile: minimal
             toolchain: nightly
             components: rustfmt, clippy, rust-src
-            override: true
 
       - name: Run cargo fmt
         uses: actions-rs/cargo@v1
@@ -164,6 +184,11 @@ jobs:
       - name: Checkout sources
         uses: actions/checkout@v2
 
+      - name: Install stable
+        uses: actions-rs/toolchain@v1
+        with:
+            toolchain: stable
+
       - name: Install latest nightly toolchain that includes Miri
         uses: actions-rs/toolchain@v1
         with:
@@ -188,11 +213,15 @@ jobs:
     - name: Checkout sources
       uses: actions/checkout@v2
 
+    - name: Install stable
+      uses: actions-rs/toolchain@v1
+      with:
+          toolchain: stable
+
     - name: Install latest nightly
       uses: actions-rs/toolchain@v1
       with:
           toolchain: nightly
-          override: true
           components: rust-src
 
     - name: Build