don't exec git

Index: dist/nimble/src/nimblepkg/options.nim
--- dist/nimble/src/nimblepkg/options.nim.orig
+++ dist/nimble/src/nimblepkg/options.nim
@@ -300,15 +300,7 @@ const
   ## sources outside the git tree of Nimble:
   git_revision_override* {.strdefine.} = ""
 
-  gitRevision* = when git_revision_override.len == 0:
-    const execResult = gorgeEx("git rev-parse HEAD")
-    when execResult[0].len > 0 and execResult[1] == QuitSuccess:
-      execResult[0]
-    else:
-      {.warning: "Couldn't determine GIT hash: " & execResult[0].}
-      "couldn't determine git hash"
-  else:
-    git_revision_override
+  gitRevision* = "couldn't determine git hash"
 
 proc writeVersion*() =
   echo("nimble v$# compiled at $# $#" %
