Bug: No LocalPreferences.toml written

The script defines JULIA_PREF_PROJECT and JULIA_PREFS:

https://github.com/JuliaParallel/JUHPC/blob/f510cfa9eb7d3fdb244b93ca7b20b9cd5487a9a0/juhpc#L190-L191

Then calls `julia_pref` many times to write to those files. However, in my install today with Julia 1.11.6+0.x64.linux.gnu, no LocalPreferences.toml file was written. I get only Project.toml.

Verbose output indicates this is because paths in `/dev/shm/$USER` are not executable.
```
[ JUHPC: Starting installation of HPC setup for juliaup, julia and HPC key packages requiring system libraries...
[ JUHPC: Installing juliaup and julia temporarily in /dev/shm/99r/juliaup...

info: downloading installer
Checking for new Julia versions
Installing Julia 1.11.6+0.x64.linux.gnu
Configured the default Julia version to be 'release'.
[ JUHPC: ... done: temporary installation completed.
[ JUHPC: Creating preferences for HPC key packages (this can take a few minutes, because some packages have to be temporarily installed)...
./juhpc: line 73: /dev/shm/99r/juliaup/bin/julia: Permission denied
./juhpc: line 73: /dev/shm/99r/juliaup/bin/julia: Permission denied
./juhpc: line 73: /dev/shm/99r/juliaup/bin/julia: Permission denied
./juhpc: line 73: /dev/shm/99r/juliaup/bin/julia: Permission denied
./juhpc: line 73: /dev/shm/99r/juliaup/bin/julia: Permission denied
./juhpc: line 73: /dev/shm/99r/juliaup/bin/julia: Permission denied
./juhpc: line 73: /dev/shm/99r/juliaup/bin/julia: Permission denied
[ JUHPC: ERROR: preferences file is missing or empty.
[ JUHPC: cleaning up temporary juliaup installation in /dev/shm/99r/juliaup.
Error running 02-script in /tmp/build/99r/juhpc.yaml
```

Manually running the juliaup installer to install to `/dev/shm/99r/juliaup` shows the file is marked executable. It's a system permission error.
```
$ ls -l /dev/shm/99r/juliaup/bin/julia
lrwxrwxrwx 1 99r 99r 38 Aug 5 16:55 /dev/shm/99r/juliaup/bin/julia -> /dev/shm/99r/juliaup/bin/julialauncher
$ ls -l /dev/shm/99r/juliaup/bin/julialauncher
-rwxr-xr-x 1 99r 99r 3307984 Apr 23 11:39 /dev/shm/99r/juliaup/bin/julialauncher
```

That causes the following line to break:
https://github.com/JuliaParallel/JUHPC/blob/f510cfa9eb7d3fdb244b93ca7b20b9cd5487a9a0/juhpc#L240C1-L240C83

My full process was:
```
export JUHPC_CUDA_HOME=$CUDA_HOME
export JUHPC_CUDA_RUNTIME_VERSION=$CRAY_CUDATOOLKIT_VERSION
export JUHPC_MPI_VENDOR=cray
export JUHPC_MPI_EXEC=srun
export JUHPC_HDF5_HOME=$HDF5_DIR

bash -l ./juhpc $prefix/juhpc $prefix/juliaup --verbose=2
```

So, the error is from my system. However, the temporary directory is not configurable as it needs to be to work around this cleanly.