bootstrap.sh: make the 'tar' invocation more portable.
bootstrap.sh: make the 'tar' invocation more portable.

Fixes #1994 (hopefully).

thoughtpolice/cabal,Heather/cabal,christiaanb/cabal,thoughtpolice/cabal,corngood/cabal,x-y-z/cabal,typelead/epm,themoritz/cabal,x-y-z/cabal,edsko/cabal,kosmikus/cabal,garetxe/cabal,kolmodin/cabal,corngood/cabal,rimmington/cabal,trskop/cabal,seereason/cabal,gridaphobe/cabal,fugyk/cabal,beni55/cabal,mydaum/cabal,ian-ross/cabal,anton-dessiatov/cabal,randen/cabal,ghc-android/cabal,headprogrammingczar/cabal,Peaker/cabal,themoritz/cabal,ian-ross/cabal,valderman/cabal,lukexi/cabal,typelead/epm,fugyk/cabal,enolan/cabal,martinvlk/cabal,Peaker/cabal,gbaz/cabal,garetxe/cabal,seereason/cabal,Heather/cabal,plumlife/cabal,thomie/cabal,triplepointfive/cabal,mmakowski/cabal,lukexi/cabal,valderman/cabal,mmakowski/cabal,trskop/cabal,rimmington/cabal,sopvop/cabal,plow-technologies/cabal,anton-dessiatov/cabal,sopvop/cabal,kolmodin/cabal,mightybyte/cabal,gridaphobe/cabal,bennofs/cabal,headprogrammingczar/cabal,plow-technologies/cabal,sopvop/cabal,ghc-android/cabal,beni55/cabal,triplepointfive/cabal,plumlife/cabal,mydaum/cabal,gbaz/cabal,bennofs/cabal,Helkafen/cabal,mightybyte/cabal,christiaanb/cabal,edsko/cabal,thomie/cabal,martinvlk/cabal,eric-stanley/cabal,eric-stanley/cabal,enolan/cabal,kosmikus/cabal,themoritz/cabal,randen/cabal,Helkafen/cabal