avoid appending lib stubs pattern to RPATH filter over and over again

[boegel]

Requires to avoid that this pattern is added again for every extension that is being installed, since EasyBlock.rpath_filter_dirs is passed down to Toolchain.prepare every time, which leads to:

== 2020-09-28 09:03:43,588 toolchain.py:973 DEBUG Combined RPATH filter: '/lib.*,/usr.*,/tmp/eb-fx9yc8f8.*,
/tmp/easybuild/build/RbundleBioconductor/3.11/foss-2020a-R-4.0.0.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,
.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,
.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.
*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,
.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,
.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,
.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,
.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,
.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*,.*/lib(64)?/stubs/?.*'

reported by @terjekv

[migueldiascosta]

lgtm

[migueldiascosta]

Going in, thanks @boegel!