[ci] AppVeyor: conda update failing with 'Invalid spec'

[jameslamb]

Description

On AppVeyor jobs, I see the following:

CondaError: Invalid spec for 'conda update': python=3.8[build=*_cp*]
Use 'conda install' instead.

(example build link)

This doesn't seem to be causing any problems... in those jobs, I still see the expected version of conda getting installed.

    python-3.8.20              |hfaddaf0_2_cpython        15.4 MB  conda-forge
    ...
    python_abi-3.8             |           7_cp38           7 KB  conda-forg

But we should still fix this.

Reproducible example

Any recent AppVeyor jobs (either MSVC or MinGW).
I'm not sure how long this has been going on.

Environment info

N/A

Additional Comments

I believe that's coming from this line:

LightGBM/.ci/test-windows.ps1

Line 69 in 72a3981

conda update -q -y conda "python=$env:PYTHON_VERSION[build=*_cp*]"