chore: remove extra slash when locating LuaJIT binary

[0x5457]

Description

When locating the LuaJIT binary within OpenResty here, an extra / is being appended to the path, resulting in an incorrect path like /usr/local/openresty//luajit/bin/luajit.

I removed the extra / when constructing the LuaJIT binary path. This ensures the correct path format without the double slash.

Checklist

• [x] I have explained the need for this PR and the problem it solves
• [x] I have explained the changes or the new features added to this PR
• [ ] I have added tests corresponding to this change
• [ ] I have updated the documentation to reflect this change
• [x] I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)