Panic with clear message when tonic_build OUT env is not set

Summary:
I was sloppy in D44322620 and:

1. Used `std::env::var` instead of `std::env::var_os`. The former does
   `OsString` to `String` conversion, which we don't care about because
   `.out_dir` expects `AsRef<Path>`.
2. Used `.unwrap` without including a message as to what expectation is
   being violated.

In D44324686 I was sloppy by leaving a needless borrow.

Reviewed By: yuyashiraki

Differential Revision: D44324930

fbshipit-source-id: f5b0152793d8537c70398ed83402fb6301b72f16

protocol-rpc/build.rs

@@ -13,10 +13,10 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         "suidcreate.proto",
     ];
     let out_env = if cfg!(fbcode_build) { "OUT" } else { "OUT_DIR" };
-    let out_dir = std::env::var_os(out_env).unwrap_or_else(|| panic!("env `{out_env}` not set"));
+    let out_dir = std::env::var_os(out_env).unwrap_or_else(|| panic!("env `{out_env}` is not set"));
 
     tonic_build::configure()
-        .out_dir(&out_dir)
+        .out_dir(out_dir)
         .compile(
             proto_files,
             // HACK: we need '.' directory for build with Buck