feat: set cache directory (#934)

crates/rattler_cache/src/lib.rs

@@ -7,9 +7,22 @@ pub mod validation;
 mod consts;
 pub use consts::{PACKAGE_CACHE_DIR, REPODATA_CACHE_DIR};
 
-/// Returns the default cache directory used by rattler.
+/// Determines the default cache directory for rattler.
+/// It first checks the environment variable `RATTLER_CACHE_DIR`.
+/// If not set, it falls back to the standard cache directory provided by `dirs::cache_dir()/rattler/cache`.
 pub fn default_cache_dir() -> anyhow::Result<PathBuf> {
-    Ok(dirs::cache_dir()
-        .ok_or_else(|| anyhow::anyhow!("could not determine cache directory for current platform"))?
-        .join("rattler/cache"))
+    std::env::var("RATTLER_CACHE_DIR")
+        .map(PathBuf::from)
+        .or_else(|_| {
+            dirs::cache_dir()
+                .ok_or_else(|| {
+                    anyhow::anyhow!("could not determine cache directory for current platform")
+                })
+                // Append `rattler/cache` to the cache directory
+                .map(|mut p| {
+                    p.push("rattler");
+                    p.push("cache");
+                    p
+                })
+        })
 }