Fixes a regression whereby, when cross-compiling, 'computed-file' would
use a cross-compiled Guile as its builder, which would fail to run.
Regression introduced in af57d1bf6c (the
problem had always been there but was hidden before behind the (not guile)
condition.)
* guix/gexp.scm (computed-file-compiler): For 'guile', pass #:target #f.
* tests/gexp.scm ("lower-object, computed-file, #:target"): New test.
