Reland "vpython: Specify interpreter to run."

This is a reland of 797d74a266

Original change's description:
> vpython: Specify interpreter to run.
> 
> Specify the interpreter to run to avoid issues with several
> python installations on Windows.
> 
> Bug: 1036606
> 
> Change-Id: Ib70d041acc52c940711460219acd8f4ad58637a9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1992608
> Reviewed-by: Anthony Polito <apolito@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

Bug: 1036606
Change-Id: Ibf6727249b40c5a9f5f877353c8d42fe66404cf8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1994215
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

vpython.bat

@@ -4,4 +4,4 @@
 :: found in the LICENSE file.
 
 call "%~dp0\cipd_bin_setup.bat" > nul 2>&1
-"%~dp0\.cipd_bin\vpython.exe" %*
+"%~dp0\.cipd_bin\vpython.exe" -vpython-interpreter "%~dp0\python.bat" %*

vpython3

@@ -47,7 +47,7 @@ if [[ $MINGW != 0 && $DEPOT_TOOLS_BOOTSTRAP_PYTHON3 != 0 ]]; then
     bootstrap_python3
   fi
   PYTHON3_BIN_RELDIR="$base_dir/$(cat $base_dir/python3_bin_reldir.txt | xargs echo)"
-  PATH="$PYTHON3_BIN_RELDIR":"$PYTHON3_BIN_RELDIR/Scripts":"$PATH"
+  exec "$base_dir/.cipd_bin/vpython3" -vpython-interpreter  "$PYTHON3_BIN_RELDIR/bin/python3" "$@"
+else
+  exec "$base_dir/.cipd_bin/vpython3" "$@"
 fi
-
-exec "$base_dir/.cipd_bin/vpython3" "$@"

vpython3.bat

@@ -9,4 +9,4 @@ for /f %%i in (%~dp0python3_bin_reldir.txt) do set PYTHON3_BIN_RELDIR=%%i
 set PATH=%~dp0%PYTHON3_BIN_RELDIR%;%PATH%
 
 call "%~dp0\cipd_bin_setup.bat" > nul 2>&1
-"%~dp0\.cipd_bin\vpython3.exe" %*
+"%~dp0\.cipd_bin\vpython3.exe" -vpython-interpreter "%~dp0\python3.bat" %*