Fix typo in specifying cmake source dir

[rwy7]

As of cmake 13.3, users must specify a source directory. Due to an old
typo in the appvyeyor config, the source directory "./" was attached to
the end of the cache name.

This PR adds back the missing space between the initial cache file and
the source directory.

Fixes: #3559

Signed-off-by: Robert Young rwy0717@gmail.com

[Leonardo2718]

LGTM 👍

[fjeremic]

The prefixes in the PR titles/commits are not very useful given that we tag the PRs anyway with the ci label. Furthermore the commit message could use some loving [1] [2]:

The first line describes the change made. It is written in the imperative mood, and should say what happens when the patch is applied. Keep it short and simple. The first line should be less than 70 characters, where reasonable, and should be written in sentence case preferably not ending in a period. Leave a blank line between the first line and the message body.

[1] https://github.com/eclipse/omr/blob/master/CONTRIBUTING.md#commit-guidelines
[2] https://chris.beams.io/posts/git-commit/

[Leonardo2718]

AppVeyor build passed previously and changes to the commit did not affect the change itself. The change only impacts AppVeyor so no further testing is required. Merging.