packaging: remap build paths out of the binary (fix $srcdir reference warning)

makepkg warned that usr/bin/peerspeak referenced $srcdir: Rust bakes source
paths into panic/backtrace metadata that survives stripping. Add
--remap-path-prefix=$srcdir=/ in build() so neither our sources nor the
vendored deps under CARGO_HOME leave the build dir embedded in the package.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packaging/PKGBUILD

@@ -38,6 +38,11 @@ build() {
   export CARGO_HOME="$srcdir/cargo-home"
   export RUSTUP_TOOLCHAIN=stable
   export CARGO_TARGET_DIR=target
+  # Strip the build directory out of paths embedded in the binary (Rust bakes
+  # source paths into panic/backtrace metadata that survives stripping), so the
+  # package doesn't reference $srcdir. One remap covers our sources and the
+  # vendored deps, since CARGO_HOME lives under $srcdir too.
+  export RUSTFLAGS="${RUSTFLAGS:-} --remap-path-prefix=$srcdir=/"
   cargo build --frozen --release --bin "$_pkgname"
 }