figlet: Fix figlet font dir (#25662)

By default, figlet looks for fonts in `/usr/local/share/figlet`, and if
it doesn't exist you get `figlet: standard: Unable to open font file`.

This fix changes the default font dir to the one installed in the
install prefix.

var/spack/repos/builtin/packages/figlet/package.py

@@ -29,3 +29,7 @@ def install(self, spec, prefix):
             install(f, prefix.man6)
 
         install_tree('./fonts', prefix.share.figlet)
+
+    @property
+    def build_targets(self):
+        return ['DEFAULTFONTDIR=' + self.prefix.share.figlet]