config: fix SPACK_DISABLE_LOCAL_CONFIG, remove $user_config_path (#27022)

There were some loose ends left in ##26735 that cause errors when
using `SPACK_DISABLE_LOCAL_CONFIG`.

- [x] Fix hard-coded `~/.spack` references in `install_test.py` and `monitor.py`

Also, if `SPACK_DISABLE_LOCAL_CONFIG` is used, there is the issue that
`$user_config_path`, when used in configuration files, makes no sense,
because there is no user config scope.

Since we already have `$user_cache_path` in configuration files, and since there
really shouldn't be *any* data stored in a configuration scope (which is what
you'd configure in `config.yaml`/`bootstrap.yaml`/etc., this just removes
`$user_config_path`.

There will *always* be a `$user_cache_path`, as Spack needs to write files, but
we shouldn't rely on the existence of a particular configuration scope in the
Spack code, as scopes are configurable, both in number and location.

- [x] Remove `$user_config_path` substitution.
- [x] Fix reference to `$user_config_path` in `etc/spack/deaults/bootstrap.yaml`
      to refer to `$user_cache_path`, which is where it was intended to be.

etc/spack/defaults/bootstrap.yaml

@@ -4,7 +4,7 @@ bootstrap:
   enable: true
   # Root directory for bootstrapping work. The software bootstrapped
   # by Spack is installed in a "store" subfolder of this root directory
-  root: $user_config_path/bootstrap
+  root: $user_cache_path/bootstrap
   # Methods that can be used to bootstrap software. Each method may or
   # may not be able to bootstrap all of the software that Spack needs,
   # depending on its type.
@@ -29,4 +29,4 @@ bootstrap:
     # By default we trust bootstrapping from sources and from binaries
     # produced on Github via the workflow
     github-actions: true
-    spack-install: true
+    spack-install: true