use whoami::fallible::hostname() instead of env variable for hostname

Summary:
I tested the `util::sys::hostname()` it return null on both linux and macOS. The test results are in D66772175 test plan.

In the past we moved from `whoami::devicename()` to reading hostname from env var. But it has the risk that envvar yields empty string.
As it was suggested on D56724499, I try using  `whoami::fallible::hostname()` which is the new version of deprecated  `whoami::hostname`.
see suggestion in:

https://www.internalfb.com/code/fbsource/[78677baeea59]/third-party/rust/vendor/whoami-1.5.2/src/api.rs?lines=88-95

`whoami::fallible::hostname()` returns `Result<String>` Then it should be safe. It catches and handles the errors.

Reviewed By: muirdm

Differential Revision: D66781930

fbshipit-source-id: 36f6a139110fd6c8136a719f15d92617f0858927

eden/scm/lib/util/Cargo.toml

@@ -25,6 +25,8 @@ sapling-rewrite-macros = { version = "0.1.0", path = "rewrite-macros" }
 shell-escape = "0.1.5"
 shellexpand = "2.1.2"
 thiserror = "2"
+tracing = { version = "0.1.40", features = ["attributes", "valuable"] }
+whoami = "1.5"
 
 [dev-dependencies]
 memmap2 = "0.5.10"

eden/scm/lib/util/src/sys.rs

@@ -16,15 +16,13 @@ pub fn hostname() -> String {
         // Also, this may fix some debugruntest issues on mac due to whoami issues.
         "test-hostname".to_owned()
     } else {
-        std::env::var_os(if cfg!(windows) {
-            "COMPUTERNAME"
-        } else if cfg!(target_os = "macos") {
-            "HOST"
-        } else {
-            "HOSTNAME"
-        })
-        .and_then(|h| h.to_str().map(|s| s.to_string()))
-        .unwrap_or("".to_owned())
+        match whoami::fallible::hostname() {
+            Ok(hostname) => hostname,
+            Err(err) => {
+                tracing::error!(?err, "error getting hostname");
+                "<UNKNOWN HOSTNAME>".to_string()
+            }
+        }
     }
 }