Pin nightly version for coverage data generation

Justfile

@@ -1,3 +1,5 @@
+nightly := "nightly-2021-06-01"
+
 # list available recipes
 default:
     @just --list --unsorted
@@ -5,12 +7,12 @@ default:
 # run integration tests with coverage
 coverage:
     cargo install grcov --git https://github.com/mozilla/grcov.git --rev c7a9b20d246a0cda812db509f206b38b3116cba4
-    rustup component add --toolchain nightly llvm-tools-preview
+    rustup component add --toolchain {{nightly}} llvm-tools-preview
 
     rm -rf *.profraw ./target/debug/coverage
 
-    RUSTFLAGS="-Zinstrument-coverage -Clink-dead-code" LLVM_PROFILE_FILE="coverage-%p-%m.profraw" cargo +nightly test --all-features
-    rustup run nightly grcov . -s . --binary-path ./target/debug/ -t html --branch --ignore-not-existing -o ./target/debug/coverage
+    RUSTFLAGS="-Zinstrument-coverage -Clink-dead-code" LLVM_PROFILE_FILE="coverage-%p-%m.profraw" cargo +{{nightly}} test --all-features
+    rustup run {{nightly}} grcov . -s . --binary-path ./target/debug/ -t html --branch --ignore-not-existing -o ./target/debug/coverage
 
     rm -f *.profraw