xilinx: ensure we chdir() back after build

mibuild/xilinx/ise.py

@@ -163,9 +163,6 @@ class XilinxISEToolchain:
 
     def build(self, platform, fragment, build_dir="build", build_name="top",
             ise_path=_default_ise_path(), source=_default_source(), run=True, mode="xst"):
-        tools.mkdir_noerror(build_dir)
-        os.chdir(build_dir)
-
         if not isinstance(fragment, _Fragment):
             fragment = fragment.get_fragment()
         platform.finalize(fragment)
@@ -174,6 +171,10 @@ class XilinxISEToolchain:
 
         vns = None
 
+        tools.mkdir_noerror(build_dir)
+        cwd = os.getcwd()
+        os.chdir(build_dir)
+        try:
             if mode == "xst" or mode == "yosys":
                 v_output = platform.get_verilog(fragment)
                 vns = v_output.ns
@@ -206,8 +207,8 @@ class XilinxISEToolchain:
                 _run_ise(build_name, ise_path, source, isemode,
                          ngdbuild_opt, self.bitgen_opt, self.ise_commands,
                          self.map_opt, self.par_opt)
-
-        os.chdir("..")
+        finally:
+            os.chdir(cwd)
 
         return vns