nix-2.17.0-zilched

• 03fbb50 FORK: Make a README note that this is a fork
• 36b3912 Revive release tarball in a limited form
• 8ed893e Make gtest optional by skipping tests in case of --disable-gtest
• 645f667 Make libmain explicitly depend on libdl
• 71e9da9 Use libsodium for SHA-2 instead of openssl, vendor MD5/SHA1
• ebc3094 Do not fail with inaccessible /proc/self/exe
• b990225 Respect NIX_FORCE_BUILD_PATH to set the path w/o sandboxing