Building master snap on non-amd64 failing to build node-sass

[Adam Collard]

As of 061f2b06c9478158bf8f38cb5177040d0dbc8348, the snap removed 'bzr' as a build dependency. This in turn removed python2 from the build environment.

It turns out that in order to build the UI on non-amd64 architectures, specifically installing node-sass, it needs python2 (!).

https://launchpadlibrarian.net/468694571/buildlog_snap_ubuntu_bionic_ppc64el_maas-master_BUILDING.txt.gz as an example.

[11/Mar/2020:20:37:00 +0000] "CONNECT github.com:443 HTTP/1.1" 200 174515 "-" "node/v12.13.0 node-sass-installer/4.13.1"
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.
error /build/maas/parts/ui/build/node_modules/node-sass: Command failed.
Exit code: 1
Command: node scripts/build.js
Arguments:
Directory: /build/maas/parts/ui/build/node_modules/node-sass
Output:
Building: /build/maas/parts/ui/npm/bin/node /build/maas/parts/ui/build/node_modules/node-gyp/bin/node-gyp.js rebuild --verbose --libsass_ext= --libsass_cflags= --libsass_ldflags= --libsass_library=
gyp info it worked if it ends with ok
gyp verb cli [
gyp verb cli '/build/maas/parts/ui/npm/bin/node',
gyp verb cli '/build/maas/parts/ui/build/node_modules/node-gyp/bin/node-gyp.js',
gyp verb cli 'rebuild',
gyp verb cli '--verbose',
gyp verb cli '--libsass_ext=',
gyp verb cli '--libsass_cflags=',
gyp verb cli '--libsass_ldflags=',
gyp verb cli '--libsass_library='
gyp verb cli ]
gyp info using node-gyp@3.8.0
gyp info using node@12.13.0 | linux | ppc64
gyp verb command rebuild []
gyp verb command clean []
gyp verb clean removing "build" directory
gyp verb command configure []
gyp verb check python checking for Python executable "python2" in the PATH
gyp verb `which` failed Error: not found: python2
gyp verb `which` failed at getNotFoundError (/build/maas/parts/ui/build/node_modules/which/which.js:13:12)
gyp verb `which` failed at F (/build/maas/parts/ui/build/node_modules/which/which.js:68:19)
gyp verb `which` failed at E (/build/maas/parts/ui/build/node_modules/which/which.js:80:29)
gyp verb `which` failed at /build/maas/parts/ui/build/node_modules/which/which.js:89:16
gyp verb `which` failed at /build/maas/parts/ui/build/node_modules/isexe/index.js:42:5
gyp verb `which` failed at /build/maas/parts/ui/build/node_modules/isexe/mode.js:8:5
gyp verb `which` failed at FSReqCallback.oncomplete (fs.js:158:21)