nexus: Use opt_prefix

Cleaning up places that can use `opt_prefix` instead of
`#{HOMEBREW_PREFIX}/opt/<packagename>`

This package had a hardcoded `/usr/local/` instead of a
Homebrew path in the .plist file.

Closes Homebrew/homebrew#22079.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/nexus.rb

@@ -28,7 +28,7 @@ class Nexus < Formula
         <string>com.sonatype.nexus</string>
         <key>ProgramArguments</key>
         <array>
-          <string>/usr/local/opt/nexus/bin/nexus</string>
+          <string>#{opt_prefix}/bin/nexus</string>
           <string>start</string>
         </array>
         <key>RunAtLoad</key>