Add TURBO_CACHE_DIR env for --cache-dir flag (#6294)

### Description

Adds the ability to set the `--cache-dir`-flag via an environment
variable named `TURBO_CACHE_DIR`.

### Testing Instructions

TURBO_CACHE_DIR=.turbo-cache turbo run build

---------

Co-authored-by: Chris Olszewski <chris.olszewski@vercel.com>
Co-authored-by: Anthony Shew <anthony.shew@vercel.com>

crates/turborepo-lib/src/cli/mod.rs

@@ -627,7 +627,7 @@ fn path_non_empty(s: &str) -> Result<Utf8PathBuf, String> {
 ])]
 pub struct RunArgs {
     /// Override the filesystem cache directory.
-    #[clap(long, value_parser = path_non_empty)]
+    #[clap(long, value_parser = path_non_empty, env = "TURBO_CACHE_DIR")]
     pub cache_dir: Option<Utf8PathBuf>,
     /// Set the number of concurrent cache operations (default 10)
     #[clap(long, default_value_t = DEFAULT_NUM_WORKERS)]

turborepo-tests/integration/tests/no-args.t

@@ -42,7 +42,7 @@ Make sure exit code is 2 when no args are passed
 
   Run Arguments:
         --cache-dir <CACHE_DIR>
-            Override the filesystem cache directory
+            Override the filesystem cache directory [env: TURBO_CACHE_DIR=]
         --cache-workers <CACHE_WORKERS>
             Set the number of concurrent cache operations (default 10) [default: 10]
         --concurrency <CONCURRENCY>

turborepo-tests/integration/tests/turbo-help.t

@@ -42,7 +42,7 @@ Test help flag
 
   Run Arguments:
         --cache-dir <CACHE_DIR>
-            Override the filesystem cache directory
+            Override the filesystem cache directory [env: TURBO_CACHE_DIR=]
         --cache-workers <CACHE_WORKERS>
             Set the number of concurrent cache operations (default 10) [default: 10]
         --concurrency <CONCURRENCY>
@@ -147,7 +147,7 @@ Test help flag
 
   Run Arguments:
         --cache-dir <CACHE_DIR>
-            Override the filesystem cache directory
+            Override the filesystem cache directory [env: TURBO_CACHE_DIR=]
         --cache-workers <CACHE_WORKERS>
             Set the number of concurrent cache operations (default 10) [default: 10]
         --concurrency <CONCURRENCY>