The upstream NRF-SDK download url and zip archive filename changed,
which was fixed with https://github.com/InfiniTimeOrg/InfiniTime/pull/2270
But the archive contents stayed the same, with the "old" folder name.
After #2270 we have basically the same docker-container as before the PR,
but the `GetNrfSdk` function of the `build.sh` script is called again during
firmware build time as the expected foldername for the SDK isn't the same as
the zip filename:
```sh
[ ! -d "$TOOLS_DIR/$NRF_SDK_VER" ] && GetNrfSdk
```
Then during the build the `buils.sh` script tries to execute `GetNrfSdk` again,
which fails as the files already exist resulting in the following error:
```
replace /opt/nRF5_SDK_15.3.0_59ac345/components/802_15_4/api/HAL/hal_atomic.h? [y]es, [n]o, [A]ll, [N]one, [r]ename: NULL
```
Fix this by reverting the `NRF_SDK_VER` to the folder name in the zip
archive and by some character replacement generate the download URL from
the above (the download is in lower-case without the `_` and `.`
characters).
Furthermore add safeguards to check after the `GetNrfSdk` call if the
expected folder is really created. Then we have an error early during
container image creation if the contents of the zip-archive are
unexpected.
Return the build status as return code from the `main` helper function.
In the process convert the handling if the file was sourced or directly
executed into an explicit if/else statement to make the intent clearer.
In case of an build error the error is now reported at the build step,
where the error happened.
Fixes: https://github.com/InfiniTimeOrg/InfiniTime/issues/1292
