pyinstaller: fix glibc dependency on gnu/linux

pyinstaller package for linux is built within the ubuntu-latest image in
github workflow. This may cause prbolem with glibc symbol versions on
older distributions, where the new symbol versions are not available.
Fix this by building on the older ubuntu version.

Closes #843

Signed-off-by: Frantisek Hrbata <frantisek.hrbata@espressif.com>

.github/workflows/build_esptool.yml

@@ -8,12 +8,12 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [macos-latest, ubuntu-latest, windows-latest]
+        os: [macos-latest, ubuntu-20.04, windows-latest]
         include:
         - os: macos-latest
           TARGET: macos
           SEPARATOR: ':'
-        - os: ubuntu-latest
+        - os: ubuntu-20.04
           TARGET: linux-amd64
           SEPARATOR: ':'
         - os: windows-latest