nanovna-saver.py: fix execution from outside source directory (#632)

nanovna-saver.py can be called from outside the source directory. The
module import path needs to be resolved relative to the source
directory, not relative to the current working directory of the
process.

Fixes: b011000 ("moved to pyscaffold directory structure")

nanovna-saver.py

@@ -17,6 +17,7 @@
 #  along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 from contextlib import suppress
+import os
 
 # noinspection PyUnresolvedReferences
 with suppress(ImportError):
@@ -29,7 +30,7 @@
 except ModuleNotFoundError:
     import sys
 
-    sys.path.append("src")
+    sys.path.append(os.path.join(os.path.dirname(__file__), "src"))
     from NanoVNASaver.__main__ import main