fix(os): use `setenv` instead of `putenv`

`setenv` doesn't copy the value that is put, which means that the
previous code had a bug where we free'd the temporary memory and the
environment was accidentally cleared right after the function finished.

core/os/os_linux.odin

@@ -458,6 +458,7 @@ foreign libc {
 	@(link_name="execvp")           _unix_execvp       :: proc(path: cstring, argv: [^]cstring) -> int ---
 	@(link_name="getenv")           _unix_getenv        :: proc(cstring) -> cstring ---
 	@(link_name="putenv")           _unix_putenv        :: proc(cstring) -> c.int ---
+	@(link_name="setenv")           _unix_setenv        :: proc(key: cstring, value: cstring, overwrite: c.int) -> c.int ---
 	@(link_name="realpath")         _unix_realpath      :: proc(path: cstring, resolved_path: rawptr) -> rawptr ---
 
 	@(link_name="exit")             _unix_exit          :: proc(status: c.int) -> ! ---
@@ -894,7 +895,10 @@ get_env :: proc(key: string, allocator := context.allocator) -> (value: string)
 set_env :: proc(key, value: string) -> Errno {
 	runtime.DEFAULT_TEMP_ALLOCATOR_TEMP_GUARD()
 	s := strings.concatenate({key, "=", value, "\x00"}, context.temp_allocator)
-	res := _unix_putenv(strings.unsafe_string_to_cstring(s))
+  key_cstring := strings.unsafe_string_to_cstring(strings.concatenate({key, "\x00"}, context.temp_allocator))
+  value_cstring := strings.unsafe_string_to_cstring(strings.concatenate({value, "\x00"}, context.temp_allocator))
+  // NOTE(GoNZooo): `setenv` instead of `putenv` because it copies both key and value more commonly
+	res := _unix_setenv(key_cstring, value_cstring, 1)
 	if res < 0 {
 		return Errno(get_last_error())
 	}