BUGFIX: passing a --oci-layout-path value with uppercase letters caused ko to fail

[inteon]

This can be reproduces very easily:

$ ko build . --oci-layout-path=AAAAA --push=false
2024/01/19 13:21:23 Using base golang:latest@sha256:5f5d61dcb58900bc57b230431b6367c900f9982b583adcabf9fa93fd0aa5544a for github.com/google/ko
2024/01/19 13:21:24 Using build config ko for github.com/google/ko
2024/01/19 13:21:24 Building github.com/google/ko for linux/amd64
2024/01/19 13:21:25 Saving ko://github.com/google/ko
2024/01/19 13:21:25 Saved ko://github.com/google/ko
Error: failed to publish images: error publishing ko://github.com/google/ko: repository can only contain the characters `abcdefghijklmnopqrstuvwxyz0123456789_-./`: AAAAA
exit status 1

On OSX especially this causes issues when you provide an absolute path.

This PR implements a very basic fix, printing the lowercase version of the path instead of the original path.

[inteon]

PTAL @imjasonh @cpanato

[cpanato]

lgtm

[inteon]

@imjasonh WDYT?