semantic-release: Check whether `$GITLAB_TOKEN` is set, prefer Vault if explicitly configured, and expand variables.

What

• fix(semantic-release): Also check whether $GITLAB_TOKEN when deciding between Vault and CI variable.

  At least the common-ci-tasks repository sets the $GITLAB_TOKEN variable. Ensure that the new logic is backwards compatible with this.

• feat(semantic-release): Change rules so that the vault input takes precedence.

  Previously, if a user provided the vault input, it was ignored if either the $SEMANTIC_RELEASE_GITLAB_TOKEN or $GITLAB_TOKEN variable was set. With this change, we'll always use the token from Vault if a Vault path is explicitly provided by the user.

• fix(semantic-release): Expand variables in the vault input.

  The input may contain variables such as ${VAULT_SECRETS_PATH}. Ensure that this works as expected.

Issue: #15