gnu: workrave: Compute source COMMIT from VERSION.

* gnu/packages/gnome.scm (workrave)[version, source]: Conventionally
generate COMMIT from VERSION, instead of the other way 'round.

gnu/packages/gnome.scm

@@ -7427,18 +7427,15 @@ configurable file renaming. ")
     (license license:gpl3)))
 
 (define-public workrave
-  (let ((commit "v1_10_21"))
   (package
     (name "workrave")
-      (version (string-map (match-lambda
-                             (#\_ #\.)
-                             (chr chr))
-                           (string-drop commit 1)))
-      (source (origin
+    (version "1.10.21")
+    (source
+     (origin
        (method git-fetch)
        (uri (git-reference
              (url "https://github.com/rcaelers/workrave.git")
-                      (commit commit)))
+             (commit (string-map (match-lambda (#\_ #\.) (chr chr)) version))))
        (file-name (git-file-name name version))
        (sha256
         (base32
@@ -7472,7 +7469,7 @@ configurable file renaming. ")
 repetitive strain injury (@dfn{RSI}).  The program frequently alerts you to take
 micro-pauses and rest breaks, and restricts you to your daily limit.")
     (home-page "http://www.workrave.org")
-      (license license:gpl3+))))
+    (license license:gpl3+)))
 
 (define-public ghex
   (package