Switch CI to using stable instead of nightly toolchain

.github/workflows/main.yml

@@ -16,7 +16,7 @@ jobs:
           - u64_backend
           - u32_backend
         toolchain:
-          - nightly
+          - stable
           - 1.41.0
     name: test
     steps:
@@ -93,7 +93,7 @@ jobs:
       fail-fast: false
       matrix:
         toolchain:
-          - nightly
+          - stable
           - 1.41.0
     name: test simple_login command-line example
     steps:
@@ -117,7 +117,7 @@ jobs:
       fail-fast: false
       matrix:
         toolchain:
-          - nightly
+          - stable
           - 1.41.0
     name: test digital_locker command-line example
     steps:
@@ -162,11 +162,11 @@ jobs:
       - name: Checkout sources
         uses: actions/checkout@v2
 
-      - name: Install nightly toolchain
+      - name: Install stable toolchain
         uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: nightly
+          toolchain: stable
           override: true
           components: rustfmt, clippy
 
@@ -184,11 +184,11 @@ jobs:
       - name: Checkout sources
         uses: actions/checkout@v2
 
-      - name: Install nightly toolchain
+      - name: Install stable toolchain
         uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: nightly
+          toolchain: stable
           override: true
           components: rustfmt, clippy