Incorrect distribution information for centos/RHEL7 tarball

1. The binary distribution listed in https://gitlab.haskell.org/haskell/ghcup/blob/master/.download-urls for GHC 8.6.5 on centos links to a non-existent file: the link points to https://downloads.haskell.org/~ghc/8.6.5/ghc-x86_64-centos7-linux.tar.xz which does not exist.

2. However, the corresponding file https://downloads.haskell.org/~ghc/8.6.5/ghc-8.6.5-x86_64-centos7-linux.tar.xz which does exist actually contains a build for GHC version 8.9.0.20190418

Steps to reproduce

1. Following haskell/ghcup#105 (closed) (patch haskell/ghcup#105 (comment 209050) applied to ghcup, tried to run ghcup install. Result is file not found.

2. Failing step 1, download https://downloads.haskell.org/~ghc/8.6.5/ghc-8.6.5-x86_64-centos7-linux.tar.xz and either install and check version, or in fact just look at the file names.

Environment

Optional:

• Operating System: Red Hat Enterprise Linux 7
• System Architecture: Linux