fix(@angular-devkit/build-angular): find ngswConfig as relative to the application root

[josephperrott]

Previously when determining the configPath for building the service worker, the config path
was loaded as a relative path to location where execution of ng build occured. Instead the
ngswConfig value provided in angular.json should be considered relative to th application
root, as is the case with the other paths provided in the configuration.

Before this change, assuming a project root of /project, and a ngswConfig value of ngsw-config.json,
the application built successfully when the exuction working directory was /project, but if the
user invoked ng build from /project/src for instance, the build failed as ngsw-config.json does not
exist at /project/src/ngsw-config.json.

[clydin]

Good catch on this one.
Unfortunately, the change will probably need to be a little more complicated since all the paths in the angular.json file should be relative to the location of the angular.json file (that location is also the workspace root). The default value for ngswConfig is ${appRoot}/ngsw-config.json with that value then being relative to the workspace root (assuming the appRoot value has not already been resolved with the workspace root).

[clydin]

Superseded by #23045

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