Add RUST_COVERAGE_IGNORE_REGEX for configurable coverage filtering

The llvm-cov export step hardcodes two ignore patterns (external/,
/tmp/). Projects often need additional filters for third-party C
code, generated files, or stdlib sources, but editing the coverage
script is not practical.

Add support for the RUST_COVERAGE_IGNORE_REGEX environment variable:
a comma-separated list of additional -ignore-filename-regex patterns
passed to llvm-cov. Users can set this via --test_env in .bazelrc:

  coverage --test_env=RUST_COVERAGE_IGNORE_REGEX=.*third_party/.*,^bazel-out/

Author: Tamas Vajk

util/collect_coverage/collect_coverage.rs

@@ -190,7 +190,21 @@ fn main() {
         .arg("-instr-profile")
         .arg(&profdata_file)
         .arg("-ignore-filename-regex=.*external/.+")
-        .arg("-ignore-filename-regex=/tmp/.+")
+        .arg("-ignore-filename-regex=/tmp/.+");
+
+    // Allow additional ignore patterns via RUST_COVERAGE_IGNORE_REGEX env var
+    // (comma-separated). This lets users filter project-specific paths like
+    // third_party/, bazel-out/, or /proc/ without patching this script.
+    if let Ok(extra) = env::var("RUST_COVERAGE_IGNORE_REGEX") {
+        for pattern in extra.split(',') {
+            let pattern = pattern.trim();
+            if !pattern.is_empty() {
+                llvm_cov_cmd.arg(format!("-ignore-filename-regex={}", pattern));
+            }
+        }
+    }
+
+    llvm_cov_cmd
         .arg(format!("-path-equivalence=.,{}", execroot.display()))
         .arg(test_binary)
         .stdout(process::Stdio::piped())