ci: keep CARGO_HOME outside the checkout so root Rust caches save

Hermit's rustup package sets CARGO_HOME to ${HERMIT_ENV}/.hermit/rust.
Swatinem/rust-cache keeps only target/ outputs for packages whose manifest
is outside the workspace root, so every registry crate looked like a
workspace member and each root-workspace cache saved an empty target/
(1303 B on main for unit-tests, rust-lint, cross, relay-e2e, and mesh
lifecycle). The exact-key hit then blocked every later save.

Set CARGO_HOME to ${HOME}/.cargo-hermit (and its bin/ on PATH) in
bin/hermit.hcl. Hermit applies that env whenever it execs cargo, so a
workflow-level CARGO_HOME export cannot do this; the environment config
fixes CI and local alike. The rust-cache contract pins it, and
bin/hermit.hcl now selects the Rust CI lanes.

Consolidate the default-feature just test-unit nextest runs into one
invocation with per-package filter clauses, so shared dependencies compile
once. Feature-flagged runs and doctests stay separate.

Signed-off-by: Elrond <28d6302a099e5225b02c4155ac4236e4912603df2ab08dbfc2f4fef08ce598c8@buzz.block.builderlab.xyz>

.github/workflows/ci.yml

@@ -48,6 +48,7 @@ jobs:
               - 'Cargo.toml'
               - 'Cargo.lock'
               - 'rust-toolchain.toml'
+              - 'bin/hermit.hcl'
               - '.config/nextest.toml'
               - 'scripts/postgres-test-*.sh'
               - 'scripts/reconcile-schema-after-pgschema.sql'