exclude /install dirs from github source export

[glensc]

this decreases github release tarball size drastically

currently each release the size grows, as you seem to store .deb and sourcecode of previous releases in github repo: https://github.com/guillaumezin/nvidiabl/tree/v0.87/install

imho you shouldn't store them there, the source only tarball can be obtained from github releases page: https://github.com/guillaumezin/nvidiabl/releases

also the install/tarball/source-only contain recursively previous releases (pff!)

anyway, this PR will exclude the /install dir from github generated tarballs: before: https://github.com/guillaumezin/nvidiabl/archive/v0.87.tar.gz (35mb) after this pr: https://github.com/glensc/nvidiabl/archive/tarball-size.tar.gz (15k)