dataconnect: fix build on macos and windows

[dconeybe]

Without this fix, the command

./gradlew :firebase-dataconnect:connectors:assembleDebug

would only succeed on Linux; on macOS it would give an error like

Could not read error output of command 'firebase-dataconnect/connectors/build/intermediates/dataconnect/debug/executable/dataconnect-v1.4.3'.
java.io.IOException: Stream Closed

or

Caused by: org.gradle.process.internal.ExecException: Process 'command 'firebase-android-sdk-main/firebase-dataconnect/connectors/build/intermediates/dataconnect/debug/executable/dataconnect-v1.4.3'' finished with non-zero exit value 126

This was because the gradle plugin for data connect was always downloading the linux binary. This PR fixes it to download the windows, macos, or linux binary, as appropriate for the host OS.

[github-actions]

Release note changes

No release note changes were detected. If you made changes that should be
present in the next release, ensure you've added an entry in the appropriate
CHANGELOG.md file(s).

[github-actions]

Test Results

 48 files   -    959   48 suites   - 959   1m 34s ⏱️ - 34m 42s
478 tests  -  5 331  477 ✅  -  5 309  1 💤  - 21  0 ❌  - 1 
956 runs   - 10 594  954 ✅  - 10 551  2 💤  - 42  0 ❌  - 1 

Results for commit 6ad30db. ± Comparison against base commit 57e64e7.

This pull request removes 5331 tests.

com.google.android.datatransport.cct.CctBackendFactoryTest ‑ create_returnCCTBackend_WhenBackendNameIsCCT
com.google.android.datatransport.cct.CctDestinationTest ‑ cctDestination_shouldOnlySupportProtoAndJson
com.google.android.datatransport.cct.CctDestinationTest ‑ cctDestination_shouldSupportProtoAndJson
com.google.android.datatransport.cct.CctTransportBackendTest ‑ decorate_whenOffline_shouldProperlyPopulateNetworkInfo
com.google.android.datatransport.cct.CctTransportBackendTest ‑ decorate_whenOnline_shouldProperlyPopulateNetworkInfo
com.google.android.datatransport.cct.CctTransportBackendTest ‑ schedule_shouldAddCookieOnPseudonymousIds
com.google.android.datatransport.cct.CctTransportBackendTest ‑ schedule_shouldDropCookieOnMixedPseudonymousIds
com.google.android.datatransport.cct.CctTransportBackendTest ‑ send_CompressedResponseIsUncompressed
com.google.android.datatransport.cct.CctTransportBackendTest ‑ send_whenBackendRedirectsMoreThan5Times_shouldOnlyRedirect4Times
com.google.android.datatransport.cct.CctTransportBackendTest ‑ send_whenBackendRedirects_shouldCorrectlyFollowTheRedirectViaPost
…

♻️ This comment has been updated with latest results.