Non-reproducibility issue in packaging

[Apteryks]

Building the package is not reproducible even with the SOURCE_DATE_EPOCH environment variable set.

The problem is setting the date explicitly: https://github.com/lsegal/yard/blob/main/yard.gemspec#L14.
This shouldn't be done (see: rubygems/rubygems#1232 (comment)).

Steps to reproduce

1. Build the YARD gem today
2. Wait for a day to pass.
3. Build the YARD gem again.

Actual Output

diff -r /gnu/store/djj5g26545667gyaw8n2zx3c4grxjbsw-ruby-yard-with-tests-0.9.25/lib/ruby/vendor_ruby/specifications/yard-0.9.25.gemspec /gnu/store/djj5g26545667gyaw8n2zx3c4grxjbsw-ruby-yard-with-tests-0.9.25-check/lib/ruby/vendor_ruby/specifications/yard-0.9.25.gemspec
12c12
<   s.date = "2020-07-15"
---
>   s.date = "2020-07-17"

Expected Output

diff -r /gnu/store/djj5g26545667gyaw8n2zx3c4grxjbsw-ruby-yard-with-tests-0.9.25/lib/ruby/vendor_ruby/specifications/yard-0.9.25.gemspec /gnu/store/djj5g26545667gyaw8n2zx3c4grxjbsw-ruby-yard-with-tests-0.9.25-check/lib/ruby/vendor_ruby/specifications/yard-0.9.25.gemspec
12c12

(shouldn't be any difference)

Environment details:

GNU Guix
Ruby 2.6.5
YARD 0.9.25

I have read the Contributing Guide.