fix commit and build date in version string

[andresilva]

The environment variables should be grabbed at compile time and not at runtime.

[andresilva]

❯ cargo run -- -v
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.19s
     Running `target/debug/subwasm -v`
subwasm v0.21.1-03d789a893e built 2024-05-21T17:47:00Z

[chevdor]

Arf good catch, thanks for the fix.