gnu: Fix build for packages inheriting from gx-guvnor-lv2.

This is a follow-up to commit 35f909d45c.

* gnu/packages/music.scm (gx-vbass-preamp-lv2)[arguments]: Make "install"
target independent from "all".
(gx-overdriver-lv2, gx-tone-mender-lv2, gx-push-pull-lv2, gx-saturator-lv2,
gx-switchless-wah-lv2, gx-slow-gear-lv2): Inherit from "gx-vbass-preamp-lv2".

gnu/packages/music.scm

@@ -2356,6 +2356,16 @@ simulation of an overdrive or distortion pedal for guitars.")
                  (base32
                   "1dzksdfrva666gpi62fd2ni9rhf18sl917f1894qr0b17pbdh9k1"))
                 (file-name (string-append name "-" version "-checkout"))))
+      (arguments
+       (substitute-keyword-arguments (package-arguments gx-guvnor-lv2)
+         ((#:phases phases)
+          `(modify-phases ,phases
+             (replace 'configure
+               (lambda _
+                 (substitute* "Makefile"
+                   (("INSTALL_DIR = .*") "INSTALL_DIR=/lib/lv2\n")
+                   (("install : all") "install :"))
+                 #t))))))
       (home-page "https://github.com/brummer10/GxVBassPreAmp.lv2")
       (synopsis "Simulation of the Vox Venue Bass 100 Pre Amp Section")
       (description "This package provides the LV2 plugin \"GxVBassPreAmp\", a
@@ -2365,7 +2375,7 @@ Section."))))
 (define-public gx-overdriver-lv2
   (let ((commit "ed71801987449414bf3adaa0dbfac68e8775f1ce")
         (revision "1"))
-    (package (inherit gx-guvnor-lv2)
+    (package (inherit gx-vbass-preamp-lv2)
       (name "gx-overdriver-lv2")
       (version (string-append "0-" revision "." (string-take commit 9)))
       (source (origin
@@ -2385,7 +2395,7 @@ overdrive effect."))))
 (define-public gx-tone-mender-lv2
   (let ((commit "b6780b4a3e4782b3ed0e5882d6788f178aed138f")
         (revision "1"))
-    (package (inherit gx-guvnor-lv2)
+    (package (inherit gx-vbass-preamp-lv2)
       (name "gx-tone-mender-lv2")
       (version (string-append "0-" revision "." (string-take commit 9)))
       (source (origin
@@ -2405,7 +2415,7 @@ clean boost effect with a 3-knob tonestack."))))
 (define-public gx-push-pull-lv2
   (let ((commit "7f76ae2068498643ac8671ee0930b13ee3fd8eb5")
         (revision "1"))
-    (package (inherit gx-guvnor-lv2)
+    (package (inherit gx-vbass-preamp-lv2)
       (name "gx-push-pull-lv2")
       (version (string-append "0-" revision "." (string-take commit 9)))
       (source (origin
@@ -2444,7 +2454,7 @@ pedal.")))
 (define-public gx-saturator-lv2
   (let ((commit "0b581ac85c515325b9f16e51937cae6e1bf81a0a")
         (revision "2"))
-    (package (inherit gx-guvnor-lv2)
+    (package (inherit gx-vbass-preamp-lv2)
       (name "gx-saturator-lv2")
       (version (string-append "0-" revision "." (string-take commit 9)))
       (source (origin
@@ -2546,7 +2556,7 @@ adjusts the amount of harmonics."))))
 (define-public gx-slow-gear-lv2
   (let ((commit "cb852e0426f4e6fe077e7f1ede73a4da335cfc5e")
         (revision "2"))
-    (package (inherit gx-guvnor-lv2)
+    (package (inherit gx-vbass-preamp-lv2)
       (name "gx-slow-gear-lv2")
       (version (string-append "0-" revision "." (string-take commit 9)))
       (source (origin
@@ -2566,7 +2576,7 @@ slow gear audio effect to produce volume swells."))))
 (define-public gx-switchless-wah-lv2
   (let ((commit "7b08691203314612999f0ce2328cdc1161cd6665")
         (revision "2"))
-    (package (inherit gx-guvnor-lv2)
+    (package (inherit gx-vbass-preamp-lv2)
       (name "gx-switchless-wah-lv2")
       (version (string-append "0-" revision "." (string-take commit 9)))
       (source (origin