Integrate gitlab-ci/fix_credentials_cache

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@cbf168eb

build:credentials: Fix error when storing credentials in cache

The credentials helper wasn't storing the credentials in the cache.
Seemingly, the echo added whitespaces at the beginning of the lines
which can't be interpreted by the git credential approve command.
Also the USERNAME was renamed to TOKEN_USER because the variable is
already filled with the system users name.