[DEPRECATION] Remove references to deprecated rand.Seed (#2650)

[lhardt]

Remove references to deprecated rand.Seed (#2650).

Context:

• From Go 1.20 (Feb 2023) onwards, math/rand.Seed is deprecated, and math/rand is already seeded by default.
• From Go 1.24 (Feb 2025?) onwards, math/rand.Seed will be a no-op, breaking the test TestPwgenNoCrand.

See method documentation in the link above for further info.

Code Changes:

• We must have our own Rand variable, so that its value can be injected in tests;
• I changed the test name of TestPwgenNoCrand to TestPwgenNoCrandFallback, to make the expected behaviour clearer;
• Shred does not need to seed, nor does TestLoadFromEnv in config_test,
  • But note that TestPwgenNoCrandFallback must restore the randomness so that TestLoadFromEnv can work properly.

This also removes the side-effect that fsutil's Shred was overwriting the seed set by pwgen.

[lhardt]

Hi! Done the linting and signoff fixes. Since this is so close to October, would you mind if I reopen the PR on Tuesday?

[dominikschulz]

@lhardt No problem at all. See you in #hacktober :)