Add environment engine calls for plugins (#12166)

# Description

This adds three engine calls: `GetEnvVar`, `GetEnvVars`, for getting
environment variables from the plugin command context, and
`GetCurrentDir` for getting the current working directory.

Plugins are now launched in the directory of their executable to try to
make improper use of the current directory without first setting it more
obvious. Plugins previously launched in whatever the current directory
of the engine was at the time the plugin command was run, but switching
to persistent plugins broke this, because they stay in whatever
directory they launched in initially.

This also fixes the `gstat` plugin to use `get_current_dir()` to
determine its repo location, which was directly affected by this
problem.

# User-Facing Changes
- Adds new engine calls (`GetEnvVar`, `GetEnvVars`, `GetCurrentDir`)
- Runs plugins in a different directory from before, in order to catch
bugs
- Plugins will have to use the new engine calls if they do filesystem
stuff to work properly

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting
- [ ] Document the working directory behavior on plugin launch
- [ ] Document the new engine calls + response type (`ValueMap`)

crates/nu-plugin/src/protocol/mod.rs

@@ -8,6 +8,8 @@ mod tests;
 #[cfg(test)]
 pub(crate) mod test_util;
 
+use std::collections::HashMap;
+
 pub use evaluated_call::EvaluatedCall;
 use nu_protocol::{
     ast::Operator, engine::Closure, Config, PipelineData, PluginSignature, RawStream, ShellError,
@@ -439,6 +441,12 @@ pub enum EngineCall<D> {
     GetConfig,
     /// Get the plugin-specific configuration (`$env.config.plugins.NAME`)
     GetPluginConfig,
+    /// Get an environment variable
+    GetEnvVar(String),
+    /// Get all environment variables
+    GetEnvVars,
+    /// Get current working directory
+    GetCurrentDir,
     /// Evaluate a closure with stream input/output
     EvalClosure {
         /// The closure to call.
@@ -462,6 +470,9 @@ impl<D> EngineCall<D> {
         match self {
             EngineCall::GetConfig => "GetConfig",
             EngineCall::GetPluginConfig => "GetPluginConfig",
+            EngineCall::GetEnvVar(_) => "GetEnv",
+            EngineCall::GetEnvVars => "GetEnvs",
+            EngineCall::GetCurrentDir => "GetCurrentDir",
             EngineCall::EvalClosure { .. } => "EvalClosure",
         }
     }
@@ -474,6 +485,7 @@ pub enum EngineCallResponse<D> {
     Error(ShellError),
     PipelineData(D),
     Config(Box<Config>),
+    ValueMap(HashMap<String, Value>),
 }
 
 impl EngineCallResponse<PipelineData> {