Fix "rbenv: command not found" error when running rbenv:install (#447)

Recently, tomo's `rbenv:install` task has started failing with this
output:

```
Installing ruby 3.3.0 -- this may take several minutes
CFLAGS=-O3 rbenv install 3.3.0 --verbose
bash: line 1: rbenv: command not found
```

This is because rbenv was recently updated[^1] to to automatically add
the `rbenv init` script to the `.bashrc` file during installation.

However, rbenv adds it to the _bottom_ of the file after the early-exit
that skips most of the `.bashrc` when it is sourced in a non-interactive
shell (i.e. tomo via SSH).

That left rbenv in an installed but broken state; hence the error.

Fix by having `rbenv:install` detect this situation and prepend the
`rbenv init` script to the _top_ of the file, where it will get loaded
for non-interactive shells such as tomo's.

[^1]: rbenv/rbenv-installer#50

lib/tomo/plugin/rbenv/tasks.rb

@@ -19,7 +19,7 @@ def run_installer
 
     def modify_bashrc
       existing_rc = remote.capture("cat", paths.bashrc, raise_on_error: false)
-      return if existing_rc.include?("rbenv init")
+      return if existing_rc.include?("rbenv init") && existing_rc.include?("$HOME/.rbenv/bin")
 
       remote.write(text: <<~BASHRC + existing_rc, to: paths.bashrc)
         if [ -d $HOME/.rbenv ]; then

test/tomo/plugin/rbenv/tasks_test.rb

@@ -61,6 +61,78 @@ def test_install_is_skipped_if_version_is_already_installed
     assert_includes(tester.stdout, "Ruby 3.2.0-rc1 is already installed")
   end
 
+  def test_install_prepends_to_bashrc_if_rbenv_init_exists_but_path_is_not_set
+    tester = configure(
+      rbenv_ruby_version: "3.1.3",
+      release_path: "/tmp/tomo/20201027184921"
+    )
+    tester.mock_script_result("cat .bashrc", stdout: <<~STDOUT)
+      # ~/.bashrc: executed by bash(1) for non-login shells.
+      # see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
+      # for examples
+
+      # If not running interactively, don't do anything
+      case $- in
+          *i*) ;;
+            *) return;;
+      esac
+
+      # Added by `rbenv init` on Sat May 25 00:47:06 UTC 2024
+      eval "$(~/.rbenv/bin/rbenv init - bash)"
+    STDOUT
+
+    tester.run_task("rbenv:install")
+    assert_match(/> .bashrc/, tester.executed_scripts[2])
+    assert_match(<<~EXPECTED.shellescape, tester.executed_scripts[2])
+      if [ -d $HOME/.rbenv ]; then
+        export PATH="$HOME/.rbenv/bin:$PATH"
+        eval "$(rbenv init -)"
+      fi
+
+      # ~/.bashrc: executed by bash(1) for non-login shells.
+      # see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
+      # for examples
+
+      # If not running interactively, don't do anything
+      case $- in
+          *i*) ;;
+            *) return;;
+      esac
+
+      # Added by `rbenv init` on Sat May 25 00:47:06 UTC 2024
+      eval "$(~/.rbenv/bin/rbenv init - bash)"
+    EXPECTED
+  end
+
+  def test_install_does_not_prepend_to_bashrc_if_rbenv_path_and_init_are_already_present
+    tester = configure(
+      rbenv_ruby_version: "3.1.3",
+      release_path: "/tmp/tomo/20201027184921"
+    )
+    tester.mock_script_result("cat .bashrc", stdout: <<~STDOUT)
+      if [ -d $HOME/.rbenv ]; then
+        export PATH="$HOME/.rbenv/bin:$PATH"
+        eval "$(rbenv init -)"
+      fi
+
+      # ~/.bashrc: executed by bash(1) for non-login shells.
+      # see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
+      # for examples
+
+      # If not running interactively, don't do anything
+      case $- in
+          *i*) ;;
+            *) return;;
+      esac
+
+      # Added by `rbenv init` on Sat May 25 00:47:06 UTC 2024
+      eval "$(~/.rbenv/bin/rbenv init - bash)"
+    STDOUT
+
+    tester.run_task("rbenv:install")
+    assert_empty(tester.executed_scripts.grep(/> .bashrc/))
+  end
+
   private
 
   def configure(settings={})