linux-initrd: 'expression->initrd' returns the complete file name.

Previously 'expression->initrd' would return the directory that contains
the 'initrd' file; now it returns the complete file name for that file.

* gnu/system/linux-initrd.scm (expression->initrd)[builder]: Change
output file name to "initrd.cpio.gz".  Tail-call 'file-append' to return
the complete file name.
* gnu/system.scm (operating-system-initrd-file): Remove 'file-append'
call.
* gnu/build/linux-initrd.scm (write-cpio-archive): Check whether OUTPUT
already has the ".gz" suffix; rename if before invoking GZIP if it does,
and otherwise after.
* gnu/system/vm.scm (expression->derivation-in-linux-vm)[builder]: Do
not append "/initrd" to #$initrd.