[et] Fix path canonicalization (flutter/engine#52274)

Previously we were getting the enclosing directory path of the `et` tool
(via the `dirname` builtin), then canonicalising the path by resolving
symlinks. This doesn't work if the `et` on the path is itself a symlink.
For example, if a user created a symlink `et` in a concrete directory at
`~/.local/bin`:

```
~/.local/bin/et -> ~/src/flutter/engines/src/flutter/bin/et
```
first we'd compute the dirname of the script:
```
~/.local/bin
```
then we'd resolve symlinks:
```
~/.local/bin
```
and incorrectly assume that was the engine directory.

Instead, we now resolve symlinks, then compute the enclosing directory.

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engine/src/flutter/bin/et

@@ -27,7 +27,7 @@ function follow_links() (
   echo "$file"
 )
 
-SCRIPT_DIR=$(follow_links "$(dirname -- "${BASH_SOURCE[0]}")")
+SCRIPT_DIR="$(dirname -- "$(follow_links "${BASH_SOURCE[0]}")")"
 ENGINE_DIR="$(cd "$SCRIPT_DIR/.."; pwd -P)"
 
 case "$(uname -s)" in