Set TMPDIR in CI and make --tempdir option work with FastGA

Two improvements for temp directory management:

1. CI: Set TMPDIR=$RUNNER_TEMP in workflow
   - GitHub Actions runners have limited /tmp space (~21GB)
   - RUNNER_TEMP is a dedicated temp directory cleaned after each job
   - This prevents 'disk space: 0 MB' errors that caused test failures

2. CLI: Make --tempdir option actually work with FastGA
   - The --tempdir flag existed but wasn't propagated to FastGA
   - Now sets TMPDIR environment variable when specified
   - FastGA uses this via Rust's tempfile crate (reads TMPDIR)
   - Logs temp directory setting when not in quiet mode

This ensures FastGA's temporary files go to the right place both
in CI (avoiding disk space issues) and when users specify --tempdir.

Author: ekg

.github/workflows/ci.yml

@@ -8,6 +8,7 @@ on:
 
 env:
   CARGO_TERM_COLOR: always
+  TMPDIR: ${{ runner.temp }}  # Use GitHub's temp directory for FastGA temporary files
 
 jobs:
   test:

src/main.rs

@@ -1415,6 +1415,14 @@ fn main() -> Result<()> {
         timing.log("start", &format!("{} | {}", timestamp, cmd_line.join(" ")));
     }
 
+    // Set TMPDIR if --tempdir is specified (FastGA uses this via Rust's tempfile crate)
+    if let Some(ref tempdir) = args.tempdir {
+        std::env::set_var("TMPDIR", tempdir);
+        if !args.quiet {
+            timing.log("tempdir", &format!("Using temp directory: {}", tempdir));
+        }
+    }
+
     // Track alignment time separately
     let mut alignment_time: Option<f64> = None;