maint: Use a pretty version string in ISO and VM images.

* gnu/system/examples/vm-image.tmpl: Use the 'GUIX_DISPLAYED_VERSION'
environment variable in 'label'.
* gnu/system/install.scm (%installation-os): Likewise.
* Makefile.am (release): Set 'GUIX_DISPLAYED_VERSION'.

Makefile.am

@@ -984,6 +984,7 @@ release: dist-with-updated-version all
 	      -v1 --no-grafts --fallback
 # Generate the ISO installation images.
 	for system in $(GUIX_SYSTEM_SUPPORTED_SYSTEMS) ; do				\
+	  GUIX_DISPLAYED_VERSION="`git describe --match=v* | sed -'es/^v//v'`" ;	\
 	  image=`$(top_builddir)/pre-inst-env						\
 	    guix system image -t iso9660						\
 	    --label="GUIX_$${system}_$(VERSION)"					\
@@ -999,6 +1000,7 @@ release: dist-with-updated-version all
 	done
 # Generate the VM images.
 	for system in $(GUIX_SYSTEM_VM_SYSTEMS) ; do					\
+	  GUIX_DISPLAYED_VERSION="`git describe --match=v* | sed -'es/^v//v'`" ;	\
 	  image=`$(top_builddir)/pre-inst-env						\
 	    guix system image -t qcow2 $(GUIX_SYSTEM_VM_IMAGE_FLAGS)			\
 	    --save-provenance								\

gnu/system/examples/vm-image.tmpl

@@ -43,7 +43,9 @@ accounts.\x1b[0m
   (keyboard-layout (keyboard-layout "us" "altgr-intl"))
 
   ;; Label for the GRUB boot menu.
-  (label (string-append "GNU Guix " (package-version guix)))
+  (label (string-append "GNU Guix "
+                        (or (getenv "GUIX_DISPLAYED_VERSION")
+                            (package-version guix))))
 
   (firmware '())
 

gnu/system/install.scm

@@ -469,7 +469,8 @@ Access documentation at any time by pressing Alt-F2.\x1b[0m
                  (bootloader grub-bootloader)
                  (targets '("/dev/sda"))))
     (label (string-append "GNU Guix installation "
-                          (package-version guix)))
+                          (or (getenv "GUIX_DISPLAYED_VERSION")
+                              (package-version guix))))
 
     ;; XXX: The AMD Radeon driver is reportedly broken, which makes kmscon
     ;; non-functional: