Difference between Xcode and git for resolving swift packages

So the background is this: I have an Xcode project that depends on a swift package that’s in a private repository on github. Of course, this requires a key to access. So far, I’ve managed to configure CI such that I can ssh into the instance and git clone the required repository for the swift package. Unfortunately when running it with xcbuild as CI does, it doesn’t work and I get this message:

static:ios distiller$ xcodebuild -showBuildSettings -workspace ./Project.xcworkspace 
    -scheme App Prod
Resolve Package Graph
Fetching [email protected]:company-uk/ProjectDependency.git
xcodebuild: error: Could not resolve package dependencies:
  Authentication failed because the credentials were rejected

To sum up, git clone will happily fetch this repo as seen here:

static:ios distiller$ git clone [email protected]:company-uk/ProjectDependency.git
Cloning into 'ProjectDependency'...
Warning: Permanently added the RSA host key for IP address '11.22.33.44' to the list of known hosts.
remote: Enumerating objects: 263, done.
remote: Counting objects: 100% (263/263), done.
remote: Compressing objects: 100% (171/171), done.
remote: Total 1335 (delta 165), reused 174 (delta 86), pack-reused 1072
Receiving objects: 100% (1335/1335), 1.11 MiB | 5.67 MiB/s, done.
Resolving deltas: 100% (681/681), done.

For a bit more context, this is running on CircleCI, set up with a Deploy key on GitHub, which has been added to the Job on CI.

Any suggestions about what might be different between the way Xcode tries to fetch dependencies and the way vanilla git does it would be great. Thanks.

Source: Swift