Add OS-specific BAZELISK_HOME (#474)

* Prefer BAZELISK_HOME with OS postfix

* Fix spelling in README.md

---------

Co-authored-by: Florian Weikert <fwe@google.com>

README.md

@@ -183,6 +183,9 @@ The following variables can be set:
 - `BAZELISK_NOJDK`
 - `BAZELISK_CLEAN`
 - `BAZELISK_GITHUB_TOKEN`
+- `BAZELISK_HOME_DARWIN`
+- `BAZELISK_HOME_LINUX`
+- `BAZELISK_HOME_WINDOWS`
 - `BAZELISK_HOME`
 - `BAZELISK_INCOMPATIBLE_FLAGS`
 - `BAZELISK_SHOW_PROGRESS`
@@ -198,6 +201,8 @@ Configuration variables are evaluated with precedence order. The preferred value
 * Variables defined in the workspace root `.bazeliskrc`
 * Variables defined in the user home `.bazeliskrc`
 
+Additionally, the Bazelisk home directory is also evaluated in precedence order. The preferred value is OS-specific e.g. `BAZELISK_HOME_LINUX`, then we fall back to `BAZELISK_HOME`.
+
 ## Requirements
 
 For ease of use, the Python version of Bazelisk is written to work with Python 2.7 and 3.x and only uses modules provided by the standard library.

core/core.go

@@ -214,7 +214,11 @@ func getBazelCommand(args []string) (string, error) {
 
 // getBazeliskHome returns the path to the Bazelisk home directory.
 func getBazeliskHome(config config.Config) (string, error) {
-	bazeliskHome := config.Get("BAZELISK_HOME")
+  bazeliskHome := config.Get("BAZELISK_HOME_" + strings.ToUpper(runtime.GOOS))
+	if len(bazeliskHome) == 0 {
+		bazeliskHome = config.Get("BAZELISK_HOME")
+	}
+
 	if len(bazeliskHome) == 0 {
 		userCacheDir, err := os.UserCacheDir()
 		if err != nil {