[ci] resolve 'invalid spec' conda errors in Windows jobs (#7087)

Author: jameslamb

.ci/test-windows.ps1

@@ -69,7 +69,7 @@ if ($env:TASK -eq "swig") {
 conda init powershell
 conda activate
 conda config --set always_yes yes --set changeps1 no
-conda update -q -y conda "python=$env:PYTHON_VERSION[build=*_cp*]"
+conda install -q -y conda "python=$env:PYTHON_VERSION[build=*_cp*]"
 
 # print output of 'conda info', to help in submitting bug reports
 Write-Output "conda info:"