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.

gnu/build/linux-initrd.scm

@@ -72,11 +72,23 @@ COMPRESS? is true, compress it using GZIP.  On success, return OUTPUT."
                                #:file->header cpio:file->cpio-header*)))
 
   (or (not compress?)
-      ;; Use '--no-name' so that gzip records neither a file name nor a time
-      ;; stamp in its output.
-      (and (zero? (system* gzip "--best" "--no-name" output))
-           (rename-file (string-append output ".gz")
-                        output))
+
+      ;; Gzip insists on adding a '.gz' suffix and does nothing if the input
+      ;; file already has that suffix.  Shuffle files around to placate it.
+      (let* ((gz-suffix? (string-suffix? ".gz" output))
+             (sans-gz    (if gz-suffix?
+                             (string-drop-right output 3)
+                             output)))
+        (when gz-suffix?
+          (rename-file output sans-gz))
+        ;; Use '--no-name' so that gzip records neither a file name nor a time
+        ;; stamp in its output.
+        (and (zero? (system* gzip "--best" "--no-name" sans-gz))
+             (begin
+               (unless gz-suffix?
+                 (rename-file (string-append output ".gz") output))
+               output)))
+
       output))
 
 (define (cache-compiled-file-name file)