chore: adapt to pixi lock v7

[aldbr]

Closes DIRACGrid/DIRAC#8533

DIRAC CI is not working after a pixi update to 0.68.0 introducing pixi lock v7.
Removing diracx-testing dependency on itself seems to solve the issue from what I can see in my local environment.
See:

$ rm pixi.lock
$ pixi lock
...
$ pixi install -e diracx-core --locked
Error:   × lock-file not up-to-date with the workspace

After removing diracx-testing from diracx-testing:

$ rm pixi.lock
$ pixi lock
...
$ pixi install -e diracx-core --locked
✔ The diracx-core environment has been installed.

[read-the-docs-community]

Documentation build overview

📚 diracx | 🛠️ Build #32667210 | 📁 Comparing 97a2960 against latest (a27e79b)

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4 files changed

± dev/how-to/write-docs/index.html
± dev/tutorials/getting-started/index.html
± user/tutorials/getting-started/index.html
± admin/how-to/install/installing/index.html