cargo: tests keep their books in /tmp when TMPDIR points elsewhere

just mutants* points TMPDIR at ~/.cache for cargo-mutants' tree copies;
the tests inherited it and synced every journal append to disk, several
times slower per mutant. Cargo's [env] reaches the tests it runs, not
cargo-mutants, so the copies stay in the cache.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

.cargo/config.toml

@@ -0,0 +1,7 @@
+# Test books (a few KB each, every journal append synced) go in the system
+# temp dir, /tmp, even when TMPDIR points elsewhere: `just mutants*` sets
+# TMPDIR to ~/.cache for cargo-mutants' multi-GB tree copies, and cargo
+# passes [env] to the tests it runs, not to cargo-mutants itself. On disk
+# the synced appends made each mutant's tests several times slower.
+[env]
+TMPDIR = { value = "/tmp", force = true }