Support compile `gix-config-value` to wasm32-wasi

[ChanTsune]

Hi! I tried to compile gix-value-config to wasm32-wasi but encountered an error during compilation with this error:

   Compiling gix-config-value v0.14.8 (/gitoxide/gix-config-value)
error[E0433]: failed to resolve: could not find `unix` in `os`
  --> gix-config-value/src/path.rs:66:30
   |
66 |                 use std::os::unix::ffi::OsStrExt;
   |                              ^^^^ could not find `unix` in `os`
   |
note: found an item that was configured out
  --> /.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/os/mod.rs:26:9
   |
26 | pub mod unix {}
   |         ^^^^
note: found an item that was configured out
  --> /.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/os/mod.rs:64:9
   |
64 | pub mod unix;
   |         ^^^^

error[E0425]: cannot find function `getpwnam` in crate `libc`
  --> gix-config-value/src/path.rs:62:38
   |
62 |             let pwd = unsafe { libc::getpwnam(cname.as_ptr()) };
   |                                      ^^^^^^^^ not found in `libc`

error[E0599]: no function or associated item named `from_bytes` found for struct `OsStr` in the current scope
  --> gix-config-value/src/path.rs:71:39
   |
71 |                 Some(std::ffi::OsStr::from_bytes(cstr.to_bytes()).into())
   |                                       ^^^^^^^^^^ function or associated item not found in `OsStr`
   |
   = help: items from traits can only be used if the trait is in scope
help: there is a method `as_bytes` with a similar name, but with different arguments
  --> /.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/os/wasi/../unix/ffi/os_str.rs:57:5
   |
57 |     fn as_bytes(&self) -> &[u8];
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: trait `OsStrExt` which provides `from_bytes` is implemented but not in scope; perhaps you want to import it
   |
10 +     use std::os::wasi::ffi::OsStrExt;
   |

Some errors have detailed explanations: E0425, E0433, E0599.
For more information about an error, try `rustc --explain E0425`.
error: could not compile `gix-config-value` (lib) due to 3 previous errors

So I added a cfg directive all(target_family = "wasm", not(target_os = "emscripten”)), because currently libc does not provide getpwnam for wasm32-wasi.

Now that gix-value-config can be compiled to wasm32-wasi, some crates that depend on gix-value-config can also be compiled to wasm32-wasi.