I have no insights about ECDSA keys but if you cannot get it to work, Julia 1.7 provides the new option of using an external git executable to bypass libgit2 and libssh2 entirely. This is the relevant release notes item:
It is now possible to use an external git executable instead of the default libgit2 library for the downloads that happen via the Git protocol by setting the environment variable
JULIA_PKG_USE_CLI_GIT=true.