Another AArch64 Linux musl build fix attempt

After testing more thoroughly on my box, it turned out that the libgcc
linking workaround should indeed fix our build errors, but on CI that
did not happen because the `RUSTFLAGS` environment variable is set,
which takes precedence over any configuration at `.cargo/config.toml`.
Therefore, let's also add the workaround to `RUSTFLAGS` on the CI
workflow definition.

This should be a satisfactory enough stopgap solution that does not
require ourselves to be pinned to a specific nightly or drop these
builds until https://github.com/rust-lang/rust/issues/128401 is sorted
out.

.cargo/config.toml

@@ -16,4 +16,9 @@ linker = "aarch64-linux-musl-gcc"
 # user-visible impacts on the final executables. See:
 # https://github.com/rust-lang/rust/issues/46651
 # https://github.com/rust-lang/compiler-builtins/issues/201
+# https://github.com/rust-lang/rust/issues/128401 (the issue most relevant to
+# our observed failures)
+# Note that these flags are not enforced on CI because CI overrides them through
+# the RUSTFLAG environment variable. Check out the CI workflow definition file at
+# .github/workflows/oxipng.yml for more info
 rustflags = ["-Clink-args=-lgcc"]

.github/workflows/oxipng.yml

@@ -55,7 +55,9 @@ jobs:
 
     env:
       CARGO_BUILD_TARGET: ${{ matrix.target }}
-      RUSTFLAGS: -Zlocation-detail=none
+      # Hopefully temporary workaround for https://github.com/rust-lang/rust/issues/128401
+      # See the comments for similar rustflag settings at .cargo/config.toml for more context
+      RUSTFLAGS: ${{ matrix.target == 'aarch64-unknown-linux-musl' && '-Zlocation-detail=none -Clink-args=-lgcc' || '-Zlocation-detail=none' }}
 
     steps:
       - name: Checkout source