Allow overriding the python interpreter used by mkosi

e.g. on CentOS 8 python3 is python 3.6. python 3.9 can be installed,
but it'll be installed as python3.9, so we need a way to override the
interpreter used by mkosi.

diff --git a/bin/mkosi b/bin/mkosi
index 487067e009100f3a199e7cd58aa5f33d4b2835bb..7d9cad88c9bc491b90136492c77616a676987bfb 100755 (executable)
--- a/bin/mkosi
+++ b/bin/mkosi
@@ -3,4 +3,4 @@
 set -e
 PYTHONPATH="$(dirname "$(dirname "$(readlink -f "${BASH_SOURCE[0]}")")")"
 export PYTHONPATH
-exec python3 -B -m mkosi "$@"
+exec ${MKOSI_INTERPRETER:-python3} -B -m mkosi "$@"