I've tested this on two different Gentoo systems on different architectures. The ruby-mysql package attempts to install files using this command: /bin/install -c -m 0755 mysql.so /var/tmp/portage/dev-ruby/mysql-ruby-2.7.3/image//usr/lib64/ruby/site_ruby/1.8/x86_64-linux make: /bin/install: Command not found As you can see, /bin/install doesn't exist; install is in /usr/bin. Not sure why it's not picking up on that.
Please search; yet another dupe of Bug 159922 :/
Sorry, I searched for "mysql-ruby install" not realizing that this was a larger problem than just this ebuild. *** This bug has been marked as a duplicate of bug 159922 ***