chore(turbopack): Fix build error when CI environment variable is not supplied (#74041)
Fixes:
```
error: environment variable `CI` not defined at compile time
--> crates/napi/src/next_api/utils.rs:130:59
|
130 | let dirty_suffix = if crate::build::GIT_CLEAN || !env!("CI").is_empty() {
| ^^^^^^^^^^
|
= help: use `std::env::var("CI")` to read the variable at run time
= note: this error originates in the macro `env` (in Nightly builds, run with -Z macro-backtrace for more info)
error: could not compile `next-swc-napi` (lib) due to 1 previous error
```