[ci] Revert fixed nightly-2021-03-23, use actual nightly

Author: notmandatory

.github/workflows/code_coverage.yml

@@ -11,7 +11,7 @@ jobs:
         uses: actions/checkout@v2
 
       - name: Set default toolchain
-        run: rustup default nightly-2021-03-23
+        run: rustup default nightly
       - name: Set profile
         run: rustup set profile minimal
 

.github/workflows/cont_integration.yml

@@ -65,7 +65,7 @@ jobs:
             target
           key: ${{ runner.os }}-cargo-test-md-docs-${{ hashFiles('**/Cargo.toml','**/Cargo.lock') }}
       - name: Set default toolchain
-        run: rustup default nightly-2021-03-23
+        run: rustup default nightly
       - name: Set profile
         run: rustup set profile minimal
       - name: Update toolchain

.github/workflows/nightly_docs.yml

@@ -18,7 +18,7 @@ jobs:
             target
           key: nightly-docs-${{ hashFiles('**/Cargo.toml','**/Cargo.lock') }}
       - name: Set default toolchain
-        run: rustup default nightly-2021-03-23
+        run: rustup default nightly
       - name: Set profile
         run: rustup set profile minimal
       - name: Update toolchain