This fixes the copy-build-system on powerpc-linux.
* guix/build/copy-build-system.scm (install)[install]: Don't clobber
'target' variable and move variable assignation into the let statement.
guix/build/copy-build-system.scm (install)[install-file]:
Read symlinks as is done in install-simple through copy-recursively.
Signed-off-by: Pierre Neidhardt <mail@ambrevar.xyz>