main/scons: fix shebang line
Before this change:
$ dabuild sh
$ sudo apk add scons >/dev/null && scons --version
env: can't execute 'python': No such file or directory
After this change:
$ dabuild sh
$ sudo apk add scons >/dev/null && scons --version
SCons by Steven Knight et al.:
script: v3.1.2.bee7caf9defd6e108fc2998a2520ddb36a967691, 2019-12-17 02:07:09, by bdeegan on octodog
engine: v3.1.2.bee7caf9defd6e108fc2998a2520ddb36a967691, 2019-12-17 02:07:09, by bdeegan on octodog
engine path: ['/usr/lib/scons/SCons']
Copyright (c) 2001 - 2019 The SCons Foundation
See also https://github.com/SCons/scons/issues/2891 and Fedora's approach around line 140 of https://apps.fedoraproject.org/packages/python3-scons/sources/spec/