feat: Use `cargo nextest` for more test parallelism

The `cargo test` command cannot parallelize across crates and it makes
tests slower than necessary:

```
$ time cargo test --all
...
real	0m8,449s
user	1m24,149s
sys	0m37,558s
```

Comparing with `nextest` the difference is clear:

```
$ time cargo nextest run --all
...
real	0m2,898s
user	0m5,922s
sys	0m2,079s
```

Signed-off-by: Wiktor Kwapisiewicz <wiktor@metacode.biz>

.env

 # Contains project specific variables
 
 # List of packages required specifically by this project
-PACMAN_PACKAGES="acl cargo-deny cargo-machete clang cmake cocogitto codespell cpio edk2-ovmf erofs-utils git jq just lychee make mkosi mold mtools openssl pkgconf podman qemu release-plz reuse ripgrep rsop rustup rust-script sbsigntools sequoia-sq shellcheck swtpm systemd-ukify tangler zstd"
+PACMAN_PACKAGES="acl cargo-deny cargo-machete cargo-nextest clang cmake cocogitto codespell cpio edk2-ovmf erofs-utils git jq just lychee make mkosi mold mtools openssl pkgconf podman qemu release-plz reuse ripgrep rsop rustup rust-script sbsigntools sequoia-sq shellcheck swtpm systemd-ukify tangler zstd"

justfile

@@ -225,7 +225,7 @@ test:
     if [[ "$ignored" == "true" ]]; then
         cargo test --all -- --ignored
     else
-        cargo test --all
+        cargo nextest run --all
         just docs
     fi