fix: allow install under read-only $HOME (#175)

* fix: set register_envs: false

DIRACOS installers do not need ~/.conda/environments.txt and the
unconditional ~/.conda creation breaks installs under a read-only
$HOME (#174).

* ci: exercise install under a read-only $HOME

Mirrors the reproducer from #174 on every basic-tests
matrix row. Runs the installer as a non-root user (root ignores
permission bits, which would mask the regression), then restores
ownership for the post-install suite since singularity/apptainer's
user-namespace mapping cannot traverse tester-owned dirs.

* ci: TEMPORARY — build against constructor fix branch

Pin constructor to chrisburr/constructor@fix/mkdir-conda-guard so the
basic-tests CI actually passes. Revert to the conda-forge release
once the upstream fix lands and ships.

Author: chrisburr

construct.yaml

@@ -16,6 +16,7 @@ post_install: create_diracosrc.sh
 keep_pkgs: false
 default_prefix: "${PWD}/diracos"
 batch_mode: true
+register_envs: false
 
 # Converting packages to be .conda files makes the installer considerably
 # smaller and faster but is computationally expensive.