build: workaround for gclient python3 issues

gclient doesn't support Python 3 yet. To workaround that problem, add an
enviroment variable to override the Python version used by ./configure.

Signed-off-by: Matheus Marchini <mmarchini@netflix.com>

PR-URL: #32140
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Christian Clauss <cclauss@me.com>

configure

@@ -7,6 +7,7 @@
 # pyenv will alert which shims are available and then will fail the build.
 _=[ 'exec' '/bin/sh' '-c' '''
 test ${TRAVIS} && exec python "$0" "$@"  # workaround for pyenv on Travis CI
+test ${FORCE_PYTHON2} && exec python2 "$0" "$@"  # workaround for gclient
 which python3.8 >/dev/null && exec python3.8 "$0" "$@"
 which python3.7 >/dev/null && exec python3.7 "$0" "$@"
 which python3.6 >/dev/null && exec python3.6 "$0" "$@"