project ssh keys should be reported as filenames, not contents

[displague]

It is difficult to mask multiline strings in GH Actions, and that handling has been incorrect to date.

Export an env var reporting a filename where the ssh key can be accessed.

The file can be created in os.Getenv('RUNNER_TEMP')

[displague]

(both public and private keys should work this way)

Since the public key does not contain newlines, we could include it in a variable.

[displague]

There are some problems with the current approach (v0.6.0):

2021/02/21 21:31:50 open /home/runner/work/_temp/id_rsa_411056339: no such file or directory

[ctreatma]

Are you still able to reproduce this issue?