Use pre Rust 1.81 compatible file-exists test

The function `std::fs::exists(...)` was stabilized in Rust 1.81, which
means it can't be used in the Debian Bookworm build. This patch swaps to
a compatible implementation leaning on the std::path::Path struct.

I'm both "upstreaming" a Debian-specific patch I had to make for the
package, and fixing the additional usage now in `main.rs`. There doesn't
seem to be any compelling reason to avoid using this function, so I
figure I should merge it into the base release.

src/api/release_attachment.rs

@@ -1,4 +1,4 @@
-use std::fs;
+use std::{fs, path};
 
 use crate::structs::Attachment;
 
@@ -16,7 +16,8 @@ pub async fn create_release_attachment(
 ) -> crate::Result<Attachment> {
     let request_url = format!("{gitea_url}/api/v1/repos/{repo}/releases/{release_id}/assets");
 
-    match fs::exists(&file) {
+    let path = path::Path::new(&file);
+    match path.try_exists() {
         Ok(true) => (),
         Ok(false) => return Err(crate::Error::NoSuchFile),
         Err(e) => {

src/main.rs

@@ -1,4 +1,5 @@
-use std::fs;
+
+use std::path;
 
 use gt_tool::cli::Args;
 use gt_tool::structs::release::{CreateReleaseOption, Release};
@@ -81,7 +82,8 @@ async fn main() -> Result<(), gt_tool::Error> {
 
             if let Some(release) = match_release_by_tag(&tag_name, release_candidates) {
                 for file in &files {
-                    match fs::exists(file) {
+                    let path = path::Path::new(&file);
+                    match path.try_exists() {
                         Ok(true) => continue,
                         Ok(false) => return Err(gt_tool::Error::NoSuchFile),
                         Err(e) => {