import: github: Filter out tags that don't look like version numbers.

* guix/import/github.scm (latest-released-version): Filter out RELEASE
if it doesn't start with digit.

guix/import/github.scm

@@ -172,7 +172,7 @@ API when using a GitHub token")
 API. This may be fixed by using an access token and setting the environment
 variable GUIX_GITHUB_TOKEN, for instance one procured from
 https://github.com/settings/tokens"))
-        (let ((proper-releases
+        (let loop ((releases
                     (filter
                      (lambda (x)
                        ;; example pre-release:
@@ -181,7 +181,7 @@ https://github.com/settings/tokens"))
                        ;; https://github.com/powertab/powertabeditor/releases
                        (not (hash-ref x "prerelease")))
                      json)))
-          (match proper-releases
+          (match releases
             (()                                   ;empty release list
              #f)
             ((release . rest)                     ;one or more releases
@@ -196,8 +196,16 @@ https://github.com/settings/tokens"))
                    (substring tag (+ name-length 1))
                    ;; some tags start with a "v" e.g. "v0.25.0"
                    ;; where some are just the version number
-                   (if (eq? (string-ref tag 0) #\v)
-                       (substring tag 1) tag)))))))))
+                   (if (string-prefix? "v" tag)
+                       (substring tag 1)
+
+                       ;; Finally, reject tags that don't start with a digit:
+                       ;; they may not represent a release.
+                       (if (and (not (string-null? tag))
+                                (char-set-contains? char-set:digit
+                                                    (string-ref tag 0)))
+                           tag
+                           (loop rest)))))))))))
 
 (define (latest-release pkg)
   "Return an <upstream-source> for the latest release of PKG."