fix!: Be aware of ForcePrepend

Previously, the value of `ForcePrepend` was not used anywhere and was not behaving as specified.
With this commit, the behavior will be as specified.

BREAKING CHANGE:
So far, `ASDF_FORCE_PREPEND` environment variable setting has been ignored, but this commit may change the behavior depending on the setting.

If it is not set, macOS behavior will not change, but Linux behavior will change.
If you want to get the same behavior as before, it is recommended to set the relevant environment variable.

Referenced Documents:
- https://github.com/asdf-vm/asdf/blob/932ac468b7c24c2adef90a293a1f7280a0074cc4/docs/manage/configuration.md#asdf_force_prepend

Background:
@andrecloutier told about this issue me in a comment on another PullRequest. Thank you!
- #2011 (comment)

Author: machupicchubeta

Diff for: internal/cli/cli.go

@@ -524,7 +524,7 @@ func envCommand(logger *log.Logger, shimmedCommand string, args []string) error
 		"ASDF_INSTALL_TYPE":    parsedVersion.Type,
 		"ASDF_INSTALL_VERSION": parsedVersion.Value,
 		"ASDF_INSTALL_PATH":    installs.InstallPath(conf, plugin, parsedVersion),
-		"PATH":                 setPath(execPaths),
+		"PATH":                 setPath(conf, execPaths),
 	}
 
 	if parsedVersion.Type != "system" {
@@ -547,8 +547,13 @@ func envCommand(logger *log.Logger, shimmedCommand string, args []string) error
 	return err
 }
 
-func setPath(paths []string) string {
-	return strings.Join(paths, ":") + ":" + os.Getenv("PATH")
+func setPath(conf config.Config, paths []string) string {
+	currentPaths := os.Getenv("PATH")
+
+	if conf.ForcePrepend {
+		return strings.Join(append(paths, currentPaths), ":")
+	}
+	return strings.Join(append([]string{currentPaths}, paths...), ":")
 }
 
 func execCommand(logger *log.Logger, command string, args []string) error {
@@ -583,7 +588,7 @@ func execCommand(logger *log.Logger, command string, args []string) error {
 		"ASDF_INSTALL_TYPE":    parsedVersion.Type,
 		"ASDF_INSTALL_VERSION": parsedVersion.Value,
 		"ASDF_INSTALL_PATH":    installs.InstallPath(conf, plugin, parsedVersion),
-		"PATH":                 setPath(execPaths),
+		"PATH":                 setPath(conf, execPaths),
 	}
 
 	if parsedVersion.Type != "system" {

Diff for: internal/cli/cli_test.go

@@ -0,0 +1,32 @@
+package cli
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/asdf-vm/asdf/internal/config"
+)
+
+func TestSetPath(t *testing.T) {
+	originalPaths := "/dummy/bin:/example/bin"
+	t.Setenv("PATH", originalPaths)
+
+	t.Run("When ForcePrepend turned on", func(t *testing.T) {
+		paths := setPath(
+			config.Config{ForcePrepend: true},
+			[]string{"/awesome/bin", "/cool/bin"},
+		)
+
+		assert.Equal(t, paths, "/awesome/bin:/cool/bin:"+originalPaths, "Then prepend path directories to the front-most part of PATH env")
+	})
+
+	t.Run("When ForcePrepend turned off", func(t *testing.T) {
+		paths := setPath(
+			config.Config{ForcePrepend: false},
+			[]string{"/awesome/bin", "/cool/bin"},
+		)
+
+		assert.Equal(t, paths, originalPaths+":/awesome/bin:/cool/bin", "Then append path directories to the hinder-most part of PATH env")
+	})
+}

Diff for: test/shim_env_command.bats

@@ -29,7 +29,7 @@ teardown() {
   echo "dummy 1.0" >"$PROJECT_DIR/.tool-versions"
   run asdf install
 
-  run asdf env dummy which dummy
+  run env "ASDF_FORCE_PREPEND=yes" asdf env dummy which dummy
   [ "$status" -eq 0 ]
   [ "$output" = "$ASDF_DIR/installs/dummy/1.0/bin/dummy" ]
 }