Adds `scripts/sign-linux-file.sh` which expectes the following variables set:
* `LINUX_GPG_PRIVATE_KEY_FILE` - Path to the GPG export of private key.
* `LINUX_GPG_PRIVATE_KEY_PASS` - Password necessary to use the private key.
Given a file it creates a file with a `.asc` suffix containing the signature:
```
> wget -q https://status-im-prs.ams3.digitaloceanspaces.com/StatusIm-210809-104514-156806-pr.tar.gz
> tar xvf StatusIm-210809-104514-156806-pr.tar.gz
StatusIm-210809-104514-156806-pr.AppImage
StatusIm-210809-104514-156806-pr.AppImage.asc
> gpg --verify StatusIm-210809-104514-156806-pr.AppImage.asc
gpg: assuming signed data in 'StatusIm-210809-104514-156806-pr.AppImage'
gpg: Signature made Mon 09 Aug 2021 12:54:49 PM CEST using RSA key ID E20B4DFD
gpg: Good signature from "Status.im Devel Signing (GPG key for signing Status.im development builds.) <devel@status.im>" [ultimate]
Primary key fingerprint: BBF0 5F92 536B ED19 30A9 FD44 009F B3BF E20B 4DFD
```
Issue: https://github.com/status-im/infra-ci/issues/25
Requires: https://github.com/status-im/status-jenkins-lib/pull/32
Signed-off-by: Jakub Sokołowski <jakub@status.im>
