Fix CI build-cache issue causing code changes to take no effect

[tstadel]

Currently if the build-cache is hit, code changes take no effect:

• without editable mode, a wheel file for haystack is created and it is installed into the python env site-packages (farm_haystack-1.1.0.dist-info) linking to the haystack location. Although there is no code installed, changes of the next commit take no effect, probably due to the interplay of python caching and build chaching.
• with editable mode there is only a farm-haystack.egg-link created in the python env pointing to the haystack location

Proposed changes:

• install haystack in editable mode in CI scripts

[ZanSara]

Great one, thank you!