Install ninja in .net8.0

[am11]

This is needed by dotnet/diagnostics where we use new native infra on lowest LTS.

[am11]

@premun, any idea about these errors?

#9 [stage-1 4/5] RUN python3 -m venv /home/helixbot/.vsts-env
#9 CANCELED
------
 > [venv 3/3] RUN python3 -m venv /venv     && . /venv/bin/activate     && HELIX_FEED=https://dnceng.pkgs.visualstudio.com/public/_packaging/helix-client-prod/pypi/simple     && pip download --no-deps --index-url $HELIX_FEED --extra-index-url  helix-scripts     && pip install --index-url  ./helix_scripts-*-py3-none-any.whl     && rm ./helix_scripts-*-py3-none-any.whl:
3.139 ERROR: You must give at least one requirement to download (see "pip help download")
3.144 WARNING: Location 'helix-scripts/pip/' is ignored: it is either a non-existing path or lacks a specific scheme.
3.246 
3.246 [notice] A new release of pip is available: 23.3.2 -> 26.1.2
3.246 [notice] To update, run: pip install --upgrade pip
------
Dockerfile:14
--------------------
  13 |     
  14 | >>> RUN python3 -m venv /venv \
  15 | >>>     && . /venv/bin/activate \
  16 | >>>     && HELIX_FEED=https://dnceng.pkgs.visualstudio.com/public/_packaging/helix-client-prod/pypi/simple \
  17 | >>>     && pip download --no-deps --index-url $HELIX_FEED --extra-index-url $PIP_INDEX_URL helix-scripts \
  18 | >>>     && pip install --index-url $PIP_INDEX_URL ./helix_scripts-*-py3-none-any.whl \
  19 | >>>     && rm ./helix_scripts-*-py3-none-any.whl
  20 |     
--------------------
ERROR: failed to solve: process "/bin/sh -c python3 -m venv /venv     && . /venv/bin/activate     && HELIX_FEED=https://dnceng.pkgs.visualstudio.com/public/_packaging/helix-client-prod/pypi/simple     && pip download --no-deps --index-url $HELIX_FEED --extra-index-url $PIP_INDEX_URL helix-scripts     && pip install --index-url $PIP_INDEX_URL ./helix_scripts-*-py3-none-any.whl     && rm ./helix_scripts-*-py3-none-any.whl" did not complete successfully: exit code: 1
12:10:33 info: ExecuteHelper[0] Retry 1/5, retrying in 1 seconds...
#0 building with "default" instance using docker driver

[premun]

cc @dkurepa

[dkurepa]

yup looking at this, it's hitting all live PRs

[am11]

Looks like PIP_INDEX_URL is empty. This part:

--extra-index-url $PIP_INDEX_URL helix-scripts 

shows up in CI logs as:

--extra-index-url  helix-scripts 

[dkurepa]

yep fixing it now

[am11]

cc @akoeplinger, net8.0 azlinux 3 leg is green. Remaining failures are unrelated to my changes.