Fix lists of paths in posix environment (#7034)

Empty paths in a colon separated list would be considered as the current
directory, so have to ensure $PATH and $LD_LIBRARY_PATH are not empty
before separating it with :

koch.nim

@@ -97,7 +97,7 @@ proc exec(cmd: string, errorcode: int = QuitFailure, additionalPath = "") =
     if not absolute.isAbsolute:
       absolute = getCurrentDir() / absolute
     echo("Adding to $PATH: ", absolute)
-    putEnv("PATH", prevPath & PathSep & absolute)
+    putEnv("PATH", (if prevPath.len > 0: prevPath & PathSep else: "") & absolute)
   echo(cmd)
   if execShellCmd(cmd) != 0: quit("FAILURE", errorcode)
   putEnv("PATH", prevPath)
@@ -402,7 +402,7 @@ proc winReleaseArch(arch: string) =
 
   template withMingw(path, body) =
     let prevPath = getEnv("PATH")
-    putEnv("PATH", path & PathSep & prevPath)
+    putEnv("PATH", (if path.len > 0: path & PathSep else: "") & prevPath)
     try:
       body
     finally: