allow Nix builds by not calling git in isGitRepo for Nimble (#23515)

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

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

lib/std/private/gitutils.nim

@@ -4,7 +4,7 @@ internal API for now, API subject to change
 
 # xxx move other git utilities here; candidate for stdlib.
 
-import std/[os, osproc, strutils, tempfiles]
+import std/[os, paths, osproc, strutils, tempfiles]
 
 when defined(nimPreviewSlimSystem):
   import std/[assertions, syncio]
@@ -32,15 +32,8 @@ template retryCall*(maxRetry = 3, backoffDuration = 1.0, call: untyped): bool =
   result
 
 proc isGitRepo*(dir: string): bool =
-  ## This command is used to get the relative path to the root of the repository.
-  ## Using this, we can verify whether a folder is a git repository by checking
-  ## whether the command success and if the output is empty.
-  let (output, status) = execCmdEx("git rev-parse --show-cdup", workingDir = dir)
-  # On Windows there will be a trailing newline on success, remove it.
-  # The value of a successful call typically won't have a whitespace (it's
-  # usually a series of ../), so we know that it's safe to unconditionally
-  # remove trailing whitespaces from the result.
-  result = status == 0 and output.strip() == ""
+  ## Avoid calling git since it depends on /bin/sh existing and fails in Nix.
+  return fileExists(dir/".git/HEAD")
 
 proc diffFiles*(path1, path2: string): tuple[output: string, same: bool] =
   ## Returns a human readable diff of files `path1`, `path2`, the exact form of