allow Nix builds by skipping isGitRepo for Nimble

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.

Reference:
status-im/nimbus-eth2#6180 (comment)

Signed-off-by: Jakub Sokołowski <jakub@status.im>

tools/deps.nim

@@ -30,15 +30,15 @@ proc cloneDependency*(destDirBase: string, url: string, commit = commitHead,
     # note: old code used `destDir / .git` but that wouldn't prevent git clone
     # from failing
     execRetry fmt"git clone -q {url} {quotedDestDir}"
-  if isGitRepo(destDir):
+  if allowBundled:
+    discard "this dependency was bundled with Nim, don't do anything"
+  elif isGitRepo(destDir):
     let oldDir = getCurrentDir()
     setCurrentDir(destDir)
     try:
       execRetry "git fetch -q"
       exec fmt"git checkout -q {commit}"
     finally:
       setCurrentDir(oldDir)
-  elif allowBundled:
-    discard "this dependency was bundled with Nim, don't do anything"
   else:
     quit "FAILURE: " & destdir & " already exists but is not a git repo"