Create datadir in post install.

* Fixes formula when bottled.

Closes #26840.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
Fix datadir.mkpath for bottled mysql server

Moved datadir.mkpath (i.e. the creation of var/mysql) to the
post_install function. This ensures that the path is created with both
compiled and bottled percona-server.

Without this fix when this formula is bottled it errors on install.

Closes #26841.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>