enable all-features in nix for CI builds

CI is running `cargo build --all-features`, so we should be passing all
the features to nix as well.

The only thing this currently affects is the jemalloc_prof feature, but if
we add any non-default features that affect nix in the future they should
also be handled correctly now.

.envrc

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-use flake
+use flake ".#${DIRENV_DEVSHELL:-default}"
 
 PATH_add bin