Pin bundler with 1.17.3 in CI (Job Failed #140924004)

Job #140924004 failed for 78ba3be1:

command -v bundler > /dev/null || gem install bundler
ERROR:  Error installing bundler:
	The last version of bundler (>= 0) to support your Ruby & RubyGems was 1.17.3. Try installing it with `gem install bundler -v 1.17.3`
	bundler requires RubyGems version >= 3.0.0. The current RubyGems version is 2.7.6. Try 'gem update --system' to update RubyGems itself.
Makefile:122: recipe for target 'bundler' failed
make: *** [bundler] Error 1

cf. #435 (closed)