fix: install one directory higher up the tree

The Shorebird archive file contains a whole file tree with `/bin`, so it
does not make sense to install the whole archive contents in `/bin`.

The following illustrates the structure of the archive:

```shell
shorebird-0.21.1
├── AUTHORS
├── COPYRIGHT
├── LICENSE-APACHE
├── LICENSE-MIT
├── NOTES_ON_CODEPUSH.md
├── README.md
├── RELEASE_NOTES.md
├── TRUSTED_TESTERS.md
├── analysis_options.yaml
├── bin
│   ├── internal
│   ├── shorebird
│   ├── shorebird.bat
│   └── shorebird.ps1
├── generate_combined_coverage.sh
├── packages
│   ├── artifact_proxy
│   ├── discord_gcp_alerts
│   ├── jwt
│   ├── redis_client
│   ├── scoped
│   ├── shorebird_cli
│   ├── shorebird_code_push_client
│   └── shorebird_code_push_protocol
├── scripts
│   ├── bootstrap.sh
│   └── patch_e2e.sh
└── third_party
    ├── README.md
    └── flutter
```

lib/utils.bash

@@ -48,7 +48,7 @@ download_release() {
 install_version() {
 	local install_type="$1"
 	local version="$2"
-	local install_path="${3%/bin}/bin"
+	local install_path="${3%/bin}"
 
 	if [ "$install_type" != "version" ]; then
 		fail "asdf-$TOOL_NAME supports release installs only"
@@ -60,8 +60,7 @@ install_version() {
 
 		local tool_cmd
 		tool_cmd="$(echo "$TOOL_TEST" | cut -d' ' -f1)"
-		chmod +x "$install_path/$tool_cmd"
-		test -x "$install_path/$tool_cmd" || fail "Expected $install_path/$tool_cmd to be executable."
+		test -x "$install_path/bin/$tool_cmd" || fail "Expected $install_path/bin/$tool_cmd to be executable."
 
 		echo "$TOOL_NAME $version installation was successful!"
 	) || (