Fix metadata for cargo binstall

[taiki-e]

#175 (comment)

[taiki-e]

bors r+

$ cargo binstall cargo-llvm-cov --manifest-path .         
13:47:44 [INFO] Installing package: 'cargo-llvm-cov'
13:47:44 [INFO] Checking for package at: 'https://github.com/taiki-e/cargo-llvm-cov/releases/download/v0.4.2/cargo-llvm-cov-x86_64-apple-darwin.tar.gz'
13:47:44 [INFO] The package will be downloaded from github.com
13:47:44 [INFO] Downloading package from: 'https://github.com/taiki-e/cargo-llvm-cov/releases/download/v0.4.2/cargo-llvm-cov-x86_64-apple-darwin.tar.gz'
13:47:45 [INFO] This will install the following binaries:
13:47:45 [INFO]   - cargo-llvm-cov (cargo-llvm-cov -> /Users/taiki/.cargo/bin/cargo-llvm-cov-v0.4.2)
13:47:45 [INFO] And create (or update) the following symlinks:
13:47:45 [INFO]   - cargo-llvm-cov (/Users/taiki/.cargo/bin/cargo-llvm-cov -> cargo-llvm-cov-v0.4.2)
13:47:45 [INFO] Do you wish to continue? yes/no
yes
13:48:02 [INFO] Installing binaries...
13:48:02 [INFO] Installation complete!

[bors]

Build succeeded:

• ci

[taiki-e]

(It seems that pkg-fmt means compression format, but it is unfortunate that commonly used extensions such as tar.gz are not supported as aliases.)