update.sh: add a shebang.

[MikeMcQuaid]

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Makes it easier for e.g. shellcheck and friends to figure out the file type.

[UniqMartin]

The file extension is already a strong hint and many tools assume Bash anyway when confronted with the “Shell Script” file type. And it's easy enough to pass --shell=bash to shellcheck.

I'm not a big fan of this because this creates the impression that this is a standalone script (it isn't) and next thing someone will propose to add executable permissions. See also 27b606d and 6106ac9.

[MikeMcQuaid]

Ok, fair enough. Need to work out how to coax my editor into doing that.

[UniqMartin]

Thanks! ❤️