fix(nix): dedupe the dev shell's PATH so mbx stays under E2BIG

flake.nix

@@ -673,6 +673,14 @@
             if [ -x "$shim" ]; then
               export PATH="$(dirname "$shim"):$PATH"
             fi
+
+            # Each nested shell (direnv, then `nix develop`, then another
+            # inside a gate) prepends this shell's ~160 store paths again,
+            # about 10 KB. mbx copies PATH into two variables at ~3.6 times
+            # its size, and Linux rejects any one variable over 128 KiB, so
+            # four nested shells failed every Cargo launch with E2BIG. Keep
+            # each directory's first entry only: lookups resolve as before.
+            PATH=$(printf '%s' "$PATH" | awk -v RS=: '!seen[$0]++ { printf "%s%s", sep, $0; sep = ":" }')
           '';
 
           env = {

quest/m0/ci/README.md

@@ -168,4 +168,3 @@ The historical samples locate candidates; they are not controlled benchmarks.
 - [The next test-CPU cuts](/quest/m0/ci/test-cost-followups.md) - the slow-test leftovers from #6987, cut at their source
 - [Biome checks every file it can parse](/quest/m0/ci/biome-whole-repo.md) - Biome runs over the whole repository and main is clean under it
 - [dispatch.sh's tests pass without KVM](/quest/m0/ci/dispatch-test-kvm.md) - `sh/dispatch-query.test.py` fakes the KVM probe instead of failing 20 tests on hosts without `/dev/kvm`
-- [mbx launches Cargo with any argument length](/quest/m0/ci/mbx-arg-max.md) - no "Argument list too long" in `sh/rs/select.test.py`'s recipe test