Revert "use python3 for goma related scripts"

This reverts commit 138ec7c5.

Reason for revert:
This might have caused the following issue:
https://bugs.chromium.org/p/chromium/issues/detail?id=1221233

Original change's description:
> use python3 for goma related scripts
>
> This uses python3 like autoninja{,.bat}.
> https://source.chromium.org/chromium/chromium/tools/depot_tools/+/main:autoninja
> https://source.chromium.org/chromium/chromium/tools/depot_tools/+/main:autoninja.bat
>
> Fixed: 1203012
> Change-Id: I4daf1892024358c5b7d88b1e32b15c82cb4411d2
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2968069
> Commit-Queue: Dirk Pranke <dpranke@google.com>
> Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
> Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
> Reviewed-by: Dirk Pranke <dpranke@google.com>

Change-Id: Ideb5004cf18c91c1404066622244a8527f5248f3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2972023
Auto-Submit: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>

goma_auth

@@ -11,4 +11,4 @@ MYPATH=$(dirname "${BASH_SOURCE[0]}")
 source "$MYPATH/cipd_bin_setup.sh"
 cipd_bin_setup &> /dev/null
 
-PYTHONDONTWRITEBYTECODE=1 exec python3 "$MYPATH/.cipd_bin/goma_auth.py" "$@"
+PYTHONDONTWRITEBYTECODE=1 exec python "$MYPATH/.cipd_bin/goma_auth.py" "$@"

goma_auth.bat

@@ -6,5 +6,5 @@
 :: See revert instructions in cipd_manifest.txt
 
 @call "%~dp0\cipd_bin_setup.bat" > nul 2>&1
-@call %scriptdir%python-bin\python3.bat %~dp0\.cipd_bin\goma_auth.py %*
+@call python.bat %~dp0\.cipd_bin\goma_auth.py %*
 exit /b

goma_ctl

@@ -11,4 +11,4 @@ MYPATH=$(dirname "${BASH_SOURCE[0]}")
 source "$MYPATH/cipd_bin_setup.sh"
 cipd_bin_setup &> /dev/null
 
-PYTHONDONTWRITEBYTECODE=1 exec python3 "$MYPATH/.cipd_bin/goma_ctl.py" "$@"
+PYTHONDONTWRITEBYTECODE=1 exec python "$MYPATH/.cipd_bin/goma_ctl.py" "$@"

goma_ctl.bat

@@ -6,5 +6,5 @@
 :: See revert instructions in cipd_manifest.txt
 
 @call "%~dp0\cipd_bin_setup.bat" > nul 2>&1
-@call %scriptdir%python-bin\python3.bat %~dp0\.cipd_bin\goma_ctl.py %*
+@call vpython.bat %~dp0\.cipd_bin\goma_ctl.py %*
 exit /b