Unable to check out SSH submodule using deploy key

Moving this thread from #116 over here

@ericsciple -- just tried the new ssh-key param but no joy .. (the SSH private key that I'm passing in via Secret is one that works with the ssh-agent hack I described earlier in this thread)

2020-03-11T20:08:31.0562996Z Current runner version: '2.165.2'
2020-03-11T20:08:31.0564725Z Prepare workflow directory
2020-03-11T20:08:31.0791957Z Prepare all required actions
2020-03-11T20:08:31.0806683Z Download action repository 'actions/checkout@master'
2020-03-11T20:08:33.0116518Z Download action repository 'sfdx-actions/setup-sfdx@v1'
2020-03-11T20:08:33.2496238Z Download action repository 'nanasess/setup-chromedriver@master'
2020-03-11T20:08:40.5559823Z Download action repository 'actions/upload-artifact@v1'
2020-03-11T20:08:41.1828303Z ##[group]Run actions/checkout@master
2020-03-11T20:08:41.1828907Z with:
2020-03-11T20:08:41.1829353Z   lfs: true
2020-03-11T20:08:41.1831532Z   ssh-key: ***
2020-03-11T20:08:41.1831924Z   submodules: true
2020-03-11T20:08:41.1832300Z   repository: <redacted>
2020-03-11T20:08:41.1832778Z   token: ***
2020-03-11T20:08:41.1833150Z   ssh-strict: true
2020-03-11T20:08:41.1833521Z   persist-credentials: true
2020-03-11T20:08:41.1833910Z   clean: true
2020-03-11T20:08:41.1834281Z   fetch-depth: 1
2020-03-11T20:08:41.1834679Z ##[endgroup]
2020-03-11T20:08:41.6235786Z Added matchers: 'checkout-git'. Problem matchers scan action output for known warning or error strings and report these inline.
2020-03-11T20:08:41.6237082Z Syncing repository: <redacted>/patronsignup
2020-03-11T20:08:41.6237958Z Working directory is '/home/runner/work/patronsignup/patronsignup'
2020-03-11T20:08:41.6310463Z [command]/usr/bin/git version
2020-03-11T20:08:41.6458784Z git version 2.25.1
2020-03-11T20:08:41.6491554Z [command]/usr/bin/git lfs version
2020-03-11T20:08:41.7979462Z git-lfs/2.10.0 (GitHub; linux amd64; go 1.13.4)
2020-03-11T20:08:41.8001760Z Deleting the contents of '/home/runner/work/patronsignup/patronsignup'
2020-03-11T20:08:41.8013376Z [command]/usr/bin/git init /home/runner/work/patronsignup/patronsignup
2020-03-11T20:08:41.8095844Z Initialized empty Git repository in /home/runner/work/patronsignup/patronsignup/.git/
2020-03-11T20:08:41.8104612Z [command]/usr/bin/git remote add origin git@github.com:<redacted>/patronsignup.git
2020-03-11T20:08:41.8155136Z [command]/usr/bin/git config --local gc.auto 0
2020-03-11T20:08:41.8200346Z [command]/usr/bin/git config --local --name-only --get-regexp core\.sshCommand
2020-03-11T20:08:41.8240012Z [command]/usr/bin/git submodule foreach --recursive git config --local --name-only --get-regexp 'core\.sshCommand' && git config --local --unset-all 'core.sshCommand' || :
2020-03-11T20:08:41.8606381Z [command]/usr/bin/git config --local --name-only --get-regexp http\.https\:\/\/github\.com\/\.extraheader
2020-03-11T20:08:41.8641874Z [command]/usr/bin/git submodule foreach --recursive git config --local --name-only --get-regexp 'http\.https\:\/\/github\.com\/\.extraheader' && git config --local --unset-all 'http.https://github.com/.extraheader' || :
2020-03-11T20:08:41.8916066Z Temporarily overriding GIT_SSH_COMMAND="/usr/bin/ssh" -i "$RUNNER_TEMP/3e0953a3-c730-442d-9693-4efc5a4164fb" -o StrictHostKeyChecking=yes -o CheckHostIP=no -o "UserKnownHostsFile=$RUNNER_TEMP/3e0953a3-c730-442d-9693-4efc5a4164fb_known_hosts"
2020-03-11T20:08:41.8921024Z [command]/usr/bin/git config --local core.sshCommand "/usr/bin/ssh" -i "$RUNNER_TEMP/3e0953a3-c730-442d-9693-4efc5a4164fb" -o StrictHostKeyChecking=yes -o CheckHostIP=no -o "UserKnownHostsFile=$RUNNER_TEMP/3e0953a3-c730-442d-9693-4efc5a4164fb_known_hosts"
2020-03-11T20:08:41.8960017Z [command]/usr/bin/git config --local http.https://github.com/.extraheader AUTHORIZATION: basic ***
2020-03-11T20:08:41.9006512Z [command]/usr/bin/git lfs install --local
2020-03-11T20:08:41.9487647Z Updated git hooks.
2020-03-11T20:08:41.9488142Z Git LFS initialized.
2020-03-11T20:08:41.9510631Z [command]/usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +c485eed0d330b13b19d081fb186d2f35836986ae:refs/remotes/origin/master
2020-03-11T20:08:42.3528109Z ERROR: Repository not found.
2020-03-11T20:08:42.3535717Z ##[error]fatal: Could not read from remote repository.
2020-03-11T20:08:42.3539415Z 
2020-03-11T20:08:42.3540237Z Please make sure you have the correct access rights
2020-03-11T20:08:42.3540662Z and the repository exists.
2020-03-11T20:08:42.3541669Z The process '/usr/bin/git' failed with exit code 128

And my workflow YAML looks like

    steps:
      - uses: actions/checkout@master
        with:
          lfs: true
          ssh-key: ${{ secrets.SFDX_COMMON_SSH_KEY }}
          submodules: true

And my .gitmodules is

[submodule "sfdx-common"]
	path = sfdx-common
	url = git@github.com:<redacted>/sfdx-common.git

Start with the workflow YAML and the .gitmodules entry shown in the report, then inspect actions/checkout's ssh-key and submodule handling around the logged git fetch. Reproduce the private-submodule checkout with a deploy key and verify that the root repository and nested submodule both fetch successfully without the repository-not-found error.