Revert "nss: 3.68.4 -> 3.79"

This reverts commit 1c76a270d2.

This breaks the Firefox 91esr build, which is apparently not compatible
with this NSS version.

/build/firefox-91.10.0/security/certverifier/OCSPVerificationTrustDomain.cpp:63:11: error: unknown type name 'SignedDigest'

Will revisit this upgrade, when we drop 91esr in favor of 102esr soon.

pkgs/development/libraries/nss/esr.nix

@@ -1,4 +1,4 @@
 import ./generic.nix {
-  version = "3.79";
-  hash = "sha256-698tapZhO2/nCtV56fmD4OlOARAXHPspmdtjPTOUpRQ=";
+  version = "3.68.4";
+  hash = "sha256-K5/T9aG0nzs7KdEgAmdPcEgRViV1b7R3KELsfDm+Fgs=";
 }