Use fixed order when loading Age identities

[k3a]

Documentation for Age says:

You can override the default lookup by:
    setting the environment variable SOPS_AGE_KEY_FILE;
    setting the SOPS_AGE_KEY environment variable;
    providing a command to output the age keys by setting the SOPS_AGE_KEY_CMD environment variable. This command can read the age recipient for which to return the private key from the SOPS_AGE_RECIPIENT environment variable.

But the original loadIdentities() code used a map which caused the order of identities to be random so sometimes SOPS_AGE_KEY_FILE would not take precedence over keys in the home directory, for example.

This fixes it by using a slice instead.