Add a WASI build about python-launcher

[brettcannon]

Turns out that nix isn't supported under WASI. https://docs.rs/wasi/0.10.2+wasi-snapshot-preview1/wasi/index.html also suggests that WASI has no execv() equivalent. Farthest this could go is to break the search code out into its own crate and make sure that can be compiled by WASI.

❯ cargo build --target wasm32-wasi --release
   Compiling python-launcher v1.0.0 (/Users/brettcannon/Repositories/brettcannon/python-launcher)
error[E0432]: unresolved import `nix::unistd`
  --> src/main.rs:38:5
   |
38 | use nix::unistd;
   |     ^^^^^^^^^^^ no `unistd` in the root

error[E0433]: failed to resolve: could not find `errno` in `nix`
  --> src/main.rs:72:54
   |
72 |                     .map_err(|message| log_exit(nix::errno::errno(), message))
   |                                                      ^^^^^ could not find `errno` in `nix`

error[E0433]: failed to resolve: could not find `errno` in `nix`
  --> src/main.rs:79:50
   |
79 |                 .map_err(|message| log_exit(nix::errno::errno(), message))
   |                                                  ^^^^^ could not find `errno` in `nix`

error[E0412]: cannot find type `Result` in crate `nix`
  --> src/main.rs:93:52
   |
93 | fn run(executable: &Path, args: &[String]) -> nix::Result<()> {
   |                                                    ^^^^^^ not found in `nix`