Customizable location for .tool-versions

### Is your feature request related to a problem? Please describe

I'd like to be able to explicitly set via the environment where the .tool-versions exists.

My current use case is an automated installation of asdf and plugins to a project workspace, where it should be kept apart from a user's own installation. The `.tools-versions` gets written and is searched for in $PWD, which means the directory the script is running from.

I would like to ensure it's written to the project space, findable by the installer, as well as by the user if the environment is set correctly. I should be able to see what the installer did with `asdf current [plugin]`.

### Describe the proposed solution

I would like location to tool-versions customizable via the environment Some options
* ASDF_DEFAULT_TOOL_VERSIONS_FILENAME is the full path name
* .tool-versions is written relative to $ASDF_DIR

I did a small test by replacing `$PWD` with `$(asdf_dir)`, for example
```
--- a/lib/commands/command-current.bash
+++ b/lib/commands/command-current.bash
@@ -10,7 +10,7 @@ plugin_current_command() {
check_if_plugin_exists "$plugin_name"

local search_path
- search_path=$PWD
+ search_path=$(asdf_dir)
``

With ASDF_DIR set to my project space, I was able to see the installed plugins with `asdf current`. This isn't an entirely correct solution since it changes the default location from where it is currently, but illustrates what I mean.

### Describe similar `asdf` features and why they are not sufficient

ASDF_DEFAULT_TOOL_VERSIONS_FILENAME is only the filename itself. asdf starts looking for it at $PWD and works its way up the tree. This may not be correct for the hands-off installation use case.

### Describe other workarounds you've considered

I've fiddled extensively with the environment variable settings
I may need to chdir in my script to get it to write things to the desired locations.

Start with lib/commands/command-current.bash and trace how asdf current searches for .tool-versions, including ASDF_DEFAULT_TOOL_VERSIONS_FILENAME and ASDF_DIR. Compare the proposed environment-variable options and verify the chosen behavior for writing and finding the file, including asdf current [plugin]; done means the project location works without changing the default behavior.

Written by the indexing model from the issue text.