Resolved an issue with "pio pkg exec" command on WIndows while executing Python scripts from a package

platformio/proc.py

@@ -185,10 +185,17 @@ def copy_pythonpath_to_osenv():
 
 
 def where_is_program(program, envpath=None):
-    env = os.environ
+    env = os.environ.copy()
     if envpath:
         env["PATH"] = envpath
 
+    # look up in $PATH
+    for bin_dir in env.get("PATH", "").split(os.pathsep):
+        if os.path.isfile(os.path.join(bin_dir, program)):
+            return os.path.join(bin_dir, program)
+        if IS_WINDOWS and os.path.isfile(os.path.join(bin_dir, "%s.exe" % program)):
+            return os.path.join(bin_dir, "%s.exe" % program)
+
     # try OS's built-in commands
     try:
         result = exec_command(["where" if IS_WINDOWS else "which", program], env=env)
@@ -197,13 +204,6 @@ def where_is_program(program, envpath=None):
     except OSError:
         pass
 
-    # look up in $PATH
-    for bin_dir in env.get("PATH", "").split(os.pathsep):
-        if os.path.isfile(os.path.join(bin_dir, program)):
-            return os.path.join(bin_dir, program)
-        if os.path.isfile(os.path.join(bin_dir, "%s.exe" % program)):
-            return os.path.join(bin_dir, "%s.exe" % program)
-
     return program