Fixes
```
umount: /tmp/nix-shell.i3xRwX/nix-test/local-overlay-store/delete-refs/stores/merged-store/nix/store: filesystem was unmounted, but failed to update userspace mount table.
make: *** [mk/lib.mk:93: tests/functional/local-overlay-store/delete-refs.sh.test] Error 16
```
in a dev shell.
Note: this previously worked before we didn't have umount in the dev
shell, so we got /run/wrappers/bin/umount.
Incorrectly high expectations lead to frustration for users who
stick around to experience how useless it is for e.g. a devShell
https://functional.cafe/@arianvp/112976284363120036:
> Flakes doesn't have eval caching. It has command line argument
> caching. It literally just stores the cli argument you passed
> in a sqlite database and yes that's as useless as it sounds
> When I discovered flakes had no expression level caching whatsoever
> I kind of felt lied to and betrayed.
Fixes
```
GEN /home/eelco/Dev/nix-master/outputs/out/share/doc/nix/manual/index.html
error: File not found: ../store/types/
┌─ release-notes/rl-next.md:60:197
│
60 │ The build hook protocol did in principle support custom ways of remote building, but that can also be accomplished with a custom service for the ssh or daemon/ssh-ng protocols, or with a custom [store type](../store/types/) i.e. `Store` subclass. <!-- we normally don't mention classes, but consider that this release note is about a library use case -->
│ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File not found: ../store/types/
Error: One or more incorrect links
2024-08-19 16:47:57 [ERROR] (mdbook::renderer): Renderer exited with non-zero return code.
2024-08-19 16:47:57 [ERROR] (mdbook::utils): Error: Rendering failed
2024-08-19 16:47:57 [ERROR] (mdbook::utils): Caused By: The "linkcheck" renderer failed
```
`make check` was reverted too soon. The hacking guide wasn't brought
up to date with the new workflow, and it's not clear how to use
meson for everything.
This reverts commit 6f3045c2a2.
This wasn't the default behaviour because:
> We don't enable this by default to avoid the mostly unnecessary work of
> performing an additional build of the package in cases where we build
> the package normally anyway, such as in our pre-merge CI.
Since we have a componentized build, we've solved the duplication.
In the new situation, building both with and without unit tests
isn't any slow than just a build with unit tests, so there's no
point in using the unit-tested build anymore.
By using the otherwise untested build, we reduce the minimum build
time towards the NixOS test, at no cost.
If you want to run all tests, build all attributes.
While we don't have any easy way to forcibly notify everyone about the
impending breakage (or forcibly migrate the users on their system),
this script enables those who do hear about the problem to migrate
their systems before they take the macOS update.
It should also enable people who only discover it after the update
when a build fails to ~fix their installs without a full reinstall.
