More specifically: call gdal-dev-env.bat as part of msvc-env.bat.

Without doing this, the a build of the qgis.sln file in MSVC fails.
Specifically, the synccrsdb project fails with:

  #>  Running crssync
  #>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\V140\Microsoft.CppCommon.targets(171,5): error MSB6006: "cmd.exe" exited with code -1073741515.

Running crssync.exe from the command line also fails:

  C:\OSGeo4W64>QGIS>ms-windows\osgeo4w\build-qgis-test-x86_64\output\bin\RelWithDebInfo\crssync.exe
  The code execution cannot proceed because gdal302.dll was not found. Reinstalling the program may fix this problem.
  The code execution cannot proceed because proj_7_1.dll was not found. Reinstalling the program may fix this problem.
  The code execution cannot proceed because gdal302.dll was not found. Reinstalling the program may fix this problem.

With this change, the configonly.bat workflow produces a qgis.sln
file that builds to completion.

The package-nightly.cmd script already called this additional .bat
file to set up the PATH to gdal and proj libraries:

  call msvc-env.bat %ARCH%
  call gdal-dev-env.bat

So it makes sense to add this call into msvc-env.bat.