`skip doctor` should validate that the Android SDK exists

[dfabulich]

To repro:

Manually break your Android SDK path, e.g. mine is in ~/Library/Android/sdk, so I renamed the sdk folder to sdkx. Then run skip doctor

Expected: skip doctor should fail, saying that the Android SDK couldn't be found

Actual: skip doctor passes with all green checkmarks, but skip checkup fails with a message like this:

[✗] Test Kotlin (2.26s)
[✗] /private/var/folders/1l/jbmc3w7947x9ljkj_260bmrc0000gn/T/902632B6-1BB4-47D5-98A1-5E55CF286A78/.build/checkouts/skip/Sources/SkipTest/XCGradleHarness.swift:320: error: -[HelloSkipTests.XCSkipTests testSkipModule] : Could not determine the dependencies of task ':HelloSkip:testDebugUnitTest'.
/private/var/folders/1l/jbmc3w7947x9ljkj_260bmrc0000gn/T/902632B6-1BB4-47D5-98A1-5E55CF286A78/Tests/HelloSkipTests/XCSkipTests.swift:19: error: -[HelloSkipTests.XCSkipTests testSkipModule] : failed - The expected test output folder did not exist, which may indicate that the gradle process encountered a build error or other issue. Missing folder: /private/var/folders/1l/jbmc3w7947x9ljkj_260bmrc0000gn/T/902632B6-1BB4-47D5-98A1-5E55CF286A78/.build/checkouts/skip/Skip/build/902632b6-1bb4-47d5-98a1-5e55cf286a78/HelloSkipTests/destination/skipstone/HelloSkip/.build/HelloSkip/test-results
[✗] Skip 1.7.0 checkup failed with 2 errors

The full log includes the actual error message:

execGradle: /opt/homebrew/bin/gradle testDebug --project-dir /private/var/folders/1l/jbmc3w7947x9ljkj_260bmrc0000gn/T/902632B6-1BB4-47D5-98A1-5E55CF286A78/.build/checkouts/skip/Skip/build/902632b6-1bb4-47d5-98a1-5e55cf286a78/HelloSkipTests/destination/skipstone --warning-mode all -PbuildDir=.build/HelloSkip --rerun-tasks --console=plain
FAILURE: Build failed with an exception.
* What went wrong:
Could not determine the dependencies of task ':HelloSkip:testDebugUnitTest'.
> SDK location not found. Define a valid SDK location with an ANDROID_HOME environment variable or by setting the sdk.dir path in your project's local properties file at '/private/var/folders/1l/jbmc3w7947x9ljkj_260bmrc0000gn/T/902632B6-1BB4-47D5-98A1-5E55CF286A78/.build/plugins/outputs/902632b6-1bb4-47d5-98a1-5e55cf286a78/HelloSkipTests/destination/skipstone/local.properties'.

SDK location not found. Define a valid SDK location with an ANDROID_HOME environment variable or by setting the sdk.dir path in your project's local properties file

[marcprux]

Yes, we should be validating the ANDROID_HOME. We do try to be clever when ANDROID_HOME is missing and hunt around for where it might be (see environmentWithDefaultToolPaths), but this isn't always right (e.g., Android Studio may have it installed somewhere else). In any case, we should validate that whatever we are trying to use is actually present.

PR welcome for this, or I can look at it when I get back from FOSDEM.