Fix disappearing HLS symlinks wrt #91

When installing a new GHC version, the corresponding
HLS symlink of that version may be accidentially removed.

Ooops.
9 jobs for master in 6 minutes and 59 seconds (queued for 2 minutes and 58 seconds)
Status Job ID Name Coverage
  Test
canceled #502959
x86_64-freebsd allowed to fail
test:freebsd:latest

canceled #502958
x86_64-freebsd
test:freebsd:recommended

00:06:59

canceled #502952
x86_64-linux
test:linux:bootstrap_script

canceled #502954
x86_64-linux allowed to fail
test:linux:latest

canceled #502953
x86_64-linux
test:linux:recommended

canceled #502955
x86_64-linux
test:linux:recommended:32bit

canceled #502951
x86_64-linux
test:linux:stack

00:06:28

canceled #502957
x86_64-darwin allowed to fail
test:mac:latest

00:06:59

canceled #502956
x86_64-darwin
test:mac:recommended

00:06:59