Bug 1605456 - add environment variable for wasi sysroot location; r=firefox-build-system-reviewers,rstewart

We often include common mozconfig fragments to turn various options on.
When doing that, environment variables are more easily turned off by
later mozconfig fragments (e.g. `build/mozconfig.no-compile` for
artifact builds) than command-line options.  Therefore, include an
alternate environment variable for identifying the location of the wasi
sysroot.

Differential Revision: https://phabricator.services.mozilla.com/D58037

--HG--
extra : moz-landing-system : lando

toolkit/moz.configure

@@ -1790,6 +1790,7 @@ lucetc = check_prog('LUCETC', ['lucetc'],
                     paths=toolchain_search_path, when=requires_wasm_sandboxing)
 
 option('--with-wasi-sysroot',
+       env='WASI_SYSROOT',
        nargs=1,
        help='Path to wasi sysroot for wasm sandboxing',
        when=requires_wasm_sandboxing)