Installations with spack inside eic-shell fails.

### Environment: (where does this bug occur, have you tried other environments)
- Which branch (often `main` for latest released):
- Which version (or `HEAD` for the most recent on git):
- Any specific OS or system where the issue occurs?
Docker version on Mac
- Any special versions of ROOT or Geant4?

### Steps to reproduce: (give a step by step account of how to trigger the bug)
```bash
. /opt/spack/share/spack/setup-env.sh
spack env activate --create --dir myenv
# The following step is needed to circumvent an earlier problem
mkdir -p /var/tmp/spack-stage
spack config add 'config:build_stage:/var/tmp/spack-stage'
spack add pigz
spack concretize --reuse

==> Concretized 1 spec:
[^] qhvp7ff pigz@2.8 build_system=makefile platform=linux os=debian13 target=x86_64_v2 %c=gcc@14.2.0
[-] fjfrh3b ^compiler-wrapper@1.1.0 build_system=generic platform=linux os=debian13 target=x86_64_v2
[e] qnoeyoe ^gcc@14.2.0+binutils+bootstrap~graphite+libsanitizer~mold~nvptx~piclibs~profiled~strip build_system=autotools build_type=RelWithDebInfo languages:='c,c++,fortran' platform=linux os=debian13 target=x86_64_v2
[^] occ2oj2 ^gcc-runtime@14.2.0 build_system=generic platform=linux os=debian13 target=x86_64_v2
[e] uvk4zqr ^glibc@2.41 build_system=autotools platform=linux os=debian13 target=x86_64_v2
[^] j67mnsi ^gmake@4.4.1~guile build_system=generic platform=linux os=debian13 target=x86_64_v2 %c=gcc@14.2.0
[^] jonei6x ^zlib-ng@2.3.3+compat+new_strategies+opt+pic+shared build_system=autotools platform=linux os=debian13 target=x86_64_v2 %c,cxx=gcc@14.2.0

spack install
```

### Expected Result: (what do you expect when you execute the steps above)
Error-free installation.

### Actual Result: (what do you get when you execute the steps above)
```
==> Warning: The following issues were ignored while updating the indices of binary caches:
FetchIndexError: Could not fetch manifest from https://ghcr.io/v2/eic/spack-v2026.06.0/manifests/index.spack, due to: GET https://ghcr.io/v2/eic/spack-v2026.06.0/manifests/index.spack returned 404: Not Found
[+] qhvp7ff pigz@2.8 /opt/software/linux-x86_64_v2/pigz-2.8-qhvp7ffnxeh2x7xhaf77fy7pofyydb7b (0s)
==> Error: 'qhvp7ffnxeh2x7xhaf77fy7pofyydb7b'
```