Remove CARGO_UNSTABLE_SPARSE_REGISTRY from GitHub actions.

The sparse registry is stable since Rust 1.68:
https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#sparse-registry.

Author: gendx

.github/workflows/benches.yml

@@ -5,7 +5,6 @@ jobs:
     runs-on: ubuntu-latest
     env:
       RUSTFLAGS: "-D warnings"
-      CARGO_UNSTABLE_SPARSE_REGISTRY: true
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@nightly

.github/workflows/format.yml

@@ -3,8 +3,6 @@ name: Formatting on nightly toolchain
 jobs:
   format:
     runs-on: ubuntu-latest
-    env:
-      CARGO_UNSTABLE_SPARSE_REGISTRY: true
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@nightly

.github/workflows/fuzzing.yml

@@ -5,7 +5,6 @@ jobs:
     runs-on: ubuntu-latest
     env:
       RUSTFLAGS: "-D warnings"
-      CARGO_UNSTABLE_SPARSE_REGISTRY: true
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@nightly

.github/workflows/lints.yml

@@ -5,7 +5,6 @@ jobs:
     runs-on: ubuntu-latest
     env:
       RUSTFLAGS: "-D warnings"
-      CARGO_UNSTABLE_SPARSE_REGISTRY: true
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable

.github/workflows/rustdoc.yml

@@ -5,7 +5,6 @@ jobs:
     runs-on: ubuntu-latest
     env:
       RUSTDOCFLAGS: "-D warnings"
-      CARGO_UNSTABLE_SPARSE_REGISTRY: true
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@nightly

.github/workflows/tests.yml

@@ -16,7 +16,6 @@ jobs:
     runs-on: ${{ matrix.os }}
     env:
       RUSTFLAGS: "-D warnings"
-      CARGO_UNSTABLE_SPARSE_REGISTRY: true
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master