Add support to the signing scripts to sign multiple browsers at the same time

Some of the signing wrappers may break when signing multiple browsers at the same time:

tools/signing/wrappers/sign-exe is outputing the signed file to /home/signing-win/last-signed-file.exe
tools/signing/wrappers/sign-mar is outputing the signed file to /home/signing-mar/last-signed-mar.mar
maybe other similar issues in other wrappers or signing scripts

If two browser are signed at the same time, the signing script may copy the signed file from the other browser.

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