Prefix matrix generation service directory sparse checkout with 'sdk'

[benbp]

Currently the sparse checkout of the service directory happens to work because it will match on the directory name, e.g.
git sparse-checkout add keyvault will end up checking out sdk/keyvault. However, some service directories (such as
in azure-sdk-for-go) are now multiple directory segments, e.g. keyvault/azkeys. This breaks the git sparse checkout
matching, so to fix the service directory can be updated to an exact path sdk/<service directory>.

[azure-sdk]

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[ghost]

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