Tweak string conversion

For correctness and MSRV compatibility.

zng/cmake.rs

@@ -1,5 +1,4 @@
 use std::env;
-use std::ffi::OsStr;
 
 pub fn build_zlib_ng(target: &str, compat: bool) {
     let mut cmake = cmake::Config::new("src/zlib-ng");
@@ -19,10 +18,8 @@ pub fn build_zlib_ng(target: &str, compat: bool) {
         // Check if we should pass on an explicit boolean value of the WITH_RVV build option.
         // See: https://github.com/zlib-ng/zlib-ng?tab=readme-ov-file#advanced-build-options
         match env::var_os("RISCV_WITH_RVV")
-            .map(OsStr::to_str)
-            .map(str::trim)
-            .map(str::to_uppercase)
-            .map(Into::into)
+            .and_then(|v| v.to_str())
+            .map(|s| s.trim().to_uppercase().as_ref())
         {
             Some("OFF" | "NO" | "FALSE" | "0") => {
                 // Force RVV off. This turns off RVV entirely, as well as the runtime check for it.