GitHub Actions: fix disable tests flag when building proj for the Windows build

[stoneLeaf]

What does this PR do?

The GitHub Action for the Windows release builds the PROJ dependency from source.
The flag passed to disable building its testing tree was renamed and thus no longer working.

The flag was renamed with PROJ version 7.0, and the version used for the build is 9.3.

Properly disabling the testing tree building speeds up the PROJ build.

References:

• gdal/.github/workflows/windows_build.yml

  Line 33 in 449d5f0

  PROJ_BRANCH: "9.3"

• https://proj.org/en/9.4/install.html#cmdoption-arg-BUILD_TESTING

What are related issues/pull requests?

N/A

Tasklist

• All CI builds and checks have passed

[coveralls]

coverage: 69.352% (-0.003%) from 69.355%
when pulling 5aa26ee on stoneLeaf:disable-tests-when-building-proj-in-windows-github-action
into 449d5f0 on OSGeo:master.