Doc: Fix append to two files in variables example

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What does this MR do and why?

The existing documentation added by Add docs for GITLAB_ENV variable (!151019 - merged) • Adam Cohen • 17.0 suggests using the following command to write contents to two files:

echo "ARCH=$(arch)" | tee >> $GITLAB_ENV build.env

However, this command results in overwriting the existing contents of $GITLAB_ENV. This MR updates the command example so it appends to both of the files and preserves the existing contents.

References

• !151019 (diffs, comment 2261898330)