Make sure the python path is configurable

build.cake

@@ -23,6 +23,7 @@ var GenApiToolPath = GetToolPath ("Microsoft.DotNet.BuildTools.GenAPI/tools/GenA
 var MDocPath = MakeAbsolute ((FilePath)"externals/api-doc-tools/bin/Release/mdoc.exe");
 var SNToolPath = GetSNToolPath (EnvironmentVariable ("SN_EXE"));
 var MSBuildToolPath = GetMSBuildToolPath (EnvironmentVariable ("MSBUILD_EXE"));
+var PythonToolPath = EnvironmentVariable ("PYTHON_EXE") ?? "python";
 
 var VERSION_ASSEMBLY = "1.59.0.0";
 var VERSION_FILE = "1.59.1.0";

cake/BuildExternals.cake

@@ -63,7 +63,7 @@ Task ("externals-init")
     .IsDependentOn ("externals-harfbuzz")
     .Does (() =>  
 {
-    RunProcess ("python", new ProcessSettings {
+    RunProcess (PythonToolPath, new ProcessSettings {
         Arguments = SKIA_PATH.CombineWithFilePath ("tools/git-sync-deps").FullPath,
         WorkingDirectory = SKIA_PATH.FullPath,
     });

cake/UtilsNative.cake

@@ -8,7 +8,7 @@ var RunGyp = new Action<string, string> ((defines, generators) =>
     Information ("\tGYP_GENERATORS = " + EnvironmentVariable ("GYP_GENERATORS"));
     Information ("\tGYP_DEFINES = " + EnvironmentVariable ("GYP_DEFINES"));
     
-    RunProcess ("python", new ProcessSettings {
+    RunProcess (PythonToolPath, new ProcessSettings {
         Arguments = SKIA_PATH.CombineWithFilePath("bin/sync-and-gyp").FullPath,
         WorkingDirectory = SKIA_PATH.FullPath,
     });