Bugfix: NSIS: Exclude Makefile* from docs

Summary:

Otherwise, the generated Makefile is included in the NSIS-installed
documentation, which can lead to non-determinism (eg, if gawk is
installed on some build VMs, but others only have mawk)

Backport of core PR14018
https://github.com/bitcoin/bitcoin/pull/14018/files

I was hoping that it would make the gitian windows build deterministic
but it is not enough. Still good to backport anyway.

Test Plan: Run the windows gitian builds.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4015