[BUG] git-2.36.0-rc0 - t6200 fails due to library/path issues

[<rsbecker@nexbridge.com>]

t6200 makes assumptions that ssh-keygen to be used is located in
/usr/local/bin. This causes problems when we have multiple OpenSSL
installations - which we do - 3 of them. Our environment supplies OpenSSL
1.1 in /usr/local-ssl1.1, with a corresponding version of ssh-agent in a
different directory. There needs to be a mechanism to override the
ssh-keygen so that tests will work in this situation.

*** RLD ERROR ***: Unresolved Text Symbol OPENSSL_add_all_algorithms_noconf
in file /usr/local/bin/ssh-keygen.

*** RLD ERROR ***: Unresolved Text Symbol EVP_MD_CTX_cleanup in file
/usr/local/bin/ssh-keygen.

*** RLD ERROR ***: Unresolved Text Symbol SSLeay in file
/usr/local/bin/ssh-keygen.

*** RLD ERROR ***: Unresolved Text Symbol EVP_MD_block_size in file
/usr/local/bin/ssh-keygen.

*** RLD ERROR ***: Unresolved Text Symbol EVP_MD_CTX_init in file
/usr/local/bin/ssh-keygen.

I do not know why the test is forcing SSH in /usr/local/bin - it is not
something coming from our environment, which has PATH set correctly.

Thanks,
Randall