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Build: Avoid non-deterministic timestamp in published builds

Make QUnit releases deterministic by embedding a known date
instead of the "current" date and time.

Adopt the standard mechanism for this based on
<https://reproducible-builds.org/docs/source-date-epoch/> which
involves an environment variable named `SOURCE_DATE_EPOCH` set
to a UNIX timestamp. This requires one line of code to then
reformat to the ISO date were prefer, but this seems worth it.

In an earlier iteration on this patch, I had the Git CLI format
it that way from the start, like so:

```
QUNIT_RELEASE_DATE="$(git show -s --format=%cd --date=short HEAD)"
//> 2021-01-09
```

Instead, we use:

```
SOURCE_DATE_EPOCH="$(git log -s --format=%at -1)"
//> 1610349865
```

... and dist-replace.js reformats it as needed.

Also add a badge to the README, inspired by
<https://github.com/jvm-repo-rebuild/reproducible-central>, but
with improved color contrast, using the same dark blue as for
the reproducible-builds.org logo, and the same green as used for
other "success" badges (e.g. CI, license check, etc.)

Fixes https://github.com/qunitjs/qunit/issues/1538.

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