Auto merge of #18971 - mbrubeck:var, r=emilio

Use env::var_os to read paths from the environment

This avoids unnecessary UTF-8 validation on OsStrings that we just pass
back to the OS.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they are code cleanup only

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components/selectors/build.rs

@@ -10,7 +10,7 @@ use std::io::{BufWriter, Write};
 use std::path::Path;
 
 fn main() {
-    let path = Path::new(&env::var("OUT_DIR").unwrap())
+    let path = Path::new(&env::var_os("OUT_DIR").unwrap())
         .join("ascii_case_insensitive_html_attributes.rs");
     let mut file = BufWriter::new(File::create(&path).unwrap());