update command: make $in(in closure body) takes cell path (#8610)

# Description

Make `$in` takes cell path in `update` command

The reason behind the change:
https://discord.com/channels/601130461678272522/615329862395101194/1088405671080370196
> when i use update on some cell path, it's almost always because i want
to start with its previous value and change it.

cc @amtoine 

# User-Facing Changes

## Before
```
open Cargo.toml | get package | update metadata.binstall.pkg-fmt {|| $in.metadata.binstall.pkg-fmt | str replace "g" "FOO"}
```

## After
```
open Cargo.toml | get package | update metadata.binstall.pkg-fmt {|| str replace "g" "FOO"}
```

If use want to access original raw, it can be accessed by parameters in
closure:
```
open Cargo.toml | get package | update metadata.binstall.pkg-fmt {|$it| $it.metadata.binstall.pkg-fmt | str replace "g" "FOO"}
```
For this reason, I don't think we need to add a flag like `--whole`

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/src/filters/update.rs

@@ -75,6 +75,11 @@ impl Command for Update {
                 example: "[[project, authors]; ['nu', ['Andrés', 'JT', 'Yehuda']]] | update authors {|row| $row.authors | str join ','}",
                 result: Some(Value::List { vals: vec![Value::Record { cols: vec!["project".into(), "authors".into()], vals: vec![Value::test_string("nu"), Value::test_string("Andrés,JT,Yehuda")], span: Span::test_data()}], span: Span::test_data()}),
             },
+            Example {
+                description: "You can also use a simple command to update 'authors' to a single string",
+                example: "[[project, authors]; ['nu', ['Andrés', 'JT', 'Yehuda']]] | update authors {|| str join ','}",
+                result: Some(Value::List { vals: vec![Value::Record { cols: vec!["project".into(), "authors".into()], vals: vec![Value::test_string("nu"), Value::test_string("Andrés,JT,Yehuda")], span: Span::test_data()}], span: Span::test_data()}),
+            }
         ]
     }
 }
@@ -118,11 +123,16 @@ fn update(
                     }
                 }
 
+                let input_at_path = match input.clone().follow_cell_path(&cell_path.members, false)
+                {
+                    Err(e) => return Value::Error { error: Box::new(e) },
+                    Ok(v) => v,
+                };
                 let output = eval_block(
                     &engine_state,
                     &mut stack,
                     &block,
-                    input.clone().into_pipeline_data(),
+                    input_at_path.into_pipeline_data(),
                     redirect_stdout,
                     redirect_stderr,
                 );