GitHub/WorkFlow: fix for CVE-2020-15228

GitHub/WorkFlow: fix for CVE-2020-15228

According the CVE-2020-15228 documented in:
 - https://github.com/advisories/GHSA-mfwh-5m23-j46w
 - https://nvd.nist.gov/vuln/detail/CVE-2020-15228

the `set-env` commands will be disabled in the near future
and should be replaced by:
    echo "FOO=BAR" >> $GITHUB_ENV

idem for `add-path`, should be replaced by:
    echo "/path/to/add" >> $GITHUB_PATH

Change-Id: I725c9ccd861a0d1580ac22491b6d716ec65973d1
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5866
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>