squisher: add jenkins compatibility

Jenkins always sets BUILD_NUMBER, which gets picked up and appended to the
ota zip file, which results in "file not existing"

Set a flag at jenkins to always override the FILE_TAG

Change-Id: Ia5d2e0ee35d0a9c16845d87b58214f88b4978658
Signed-off-by: Alexander Martinz <eviscerationls@gmail.com>