Adds Python var and defaults to python (#28102)

Co-authored-by: Silvio Guiso <v-siguis@microsoft.com>

src/mscorlib/System.Private.CoreLib.csproj

@@ -639,6 +639,7 @@
     <!-- Compiler Definition Verification -->
     <PropertyGroup>
       <CMakeDefinitionSaveFile>$(IntermediateOutputPath)..\cmake.definitions</CMakeDefinitionSaveFile>
+      <PYTHON Condition=" '$(PYTHON)' == '' ">python</PYTHON>
     </PropertyGroup>
     <Exec Command="&quot;$(PYTHON)&quot; $(MSBuildThisFileDirectory)..\scripts\check-definitions.py &quot;$(CMakeDefinitionSaveFile)&quot; &quot;$(DefineConstants)&quot; &quot;$(IgnoreDefineConstants)&quot; " />
   </Target>
@@ -670,4 +671,4 @@
   </ItemGroup>
   <Import Project="ILLink.targets" />
   <Import Project="GenerateCompilerResponseFile.targets" />
-</Project>
+</Project>

sync.sh

@@ -1,5 +1,24 @@
 #!/usr/bin/env bash
 
+# resolve python-version to use
+if [ "$PYTHON" == "" ] ; then
+    if ! PYTHON=$(command -v python2.7 || command -v python2 || command -v python || command -v python3)
+    then
+       echo "Unable to locate build-dependency python!" 1>&2
+       exit 1
+    fi
+fi
+
+# validate python-dependency
+# useful in case of explicitly set option.
+if ! command -v $PYTHON > /dev/null
+then
+   echo "Unable to locate build-dependency python ($PYTHON)!" 1>&2
+   exit 1
+fi
+
+export PYTHON
+
 usage()
 {
     echo "Usage: sync [-p]"