ci(docker): fix repo mount path for pkgmgr as base layer of Infinito.Nexus

Standardize Docker/CI/test environments to mount pkgmgr at /opt/src/pkgmgr.
This makes the layering explicit: pkgmgr is the lower-level foundation used by
Infinito.Nexus.

Infra-only change (Docker, CI, shell scripts). No runtime or Nix semantics changed.

https://chatgpt.com/share/69427fe7-e288-800f-90a4-c1c3c11a8484

.github/workflows/test-virgin-user.yml

@@ -31,8 +31,8 @@ jobs:
           set -euo pipefail
 
           docker run --rm \
-            -v "$PWD":/src \
-            -w /src \
+            -v "$PWD":/opt/src/pkgmgr \
+            -w /opt/src/pkgmgr \
             "pkgmgr-${{ matrix.distro }}-virgin" \
             bash -lc '
               set -euo pipefail
@@ -42,7 +42,7 @@ jobs:
               useradd -m dev
               echo "dev ALL=(ALL) NOPASSWD: ALL" > /etc/sudoers.d/dev
               chmod 0440 /etc/sudoers.d/dev
-              chown -R dev:dev /src
+              chown -R dev:dev /opt/src/pkgmgr
 
               mkdir -p /nix/store /nix/var/nix /nix/var/log/nix /nix/var/nix/profiles
               chown -R dev:dev /nix
@@ -51,7 +51,7 @@ jobs:
 
               sudo -H -u dev env HOME=/home/dev PKGMGR_DISABLE_NIX_FLAKE_INSTALLER=1 bash -lc "
                 set -euo pipefail
-                cd /src
+                cd /opt/src/pkgmgr
 
                 make setup-venv
                 . \"\$HOME/.venvs/pkgmgr/bin/activate\"
@@ -59,6 +59,6 @@ jobs:
                 pkgmgr version pkgmgr
 
                 export NIX_REMOTE=local
-                nix run /src#pkgmgr -- version pkgmgr
+                nix run /opt/src/pkgmgr#pkgmgr -- version pkgmgr
               "
             '