Because `isGitRepo()` call requires `/bin/sh` it will always fail when
building Nim in a Nix build sandbox, and the check doesn't even make
sense if Nix already provides Nimble source code.
Since for Nimble `allowBundled` is set to `true` this effectlvely does
not change behavior for normal builds, but does avoid ugly hacks when
building in Nix which lacks `/bin/sh` and fails to call `git`.
Reference:
*
https://github.com/status-im/nimbus-eth2/pull/6180#discussion_r1570237858
Signed-off-by: Jakub Sokołowski <jakub@status.im>