Taskfile: build x86_64-linux binaries using musl

Should fix dependency on glibc 2.5 otherwise:

```
/Users/slimsag/.asdf/installs/golang/1.18/go/pkg/tool/darwin_amd64/link: running zig failed: exit status 1
ld.lld: error: undefined symbol: getrandom
```

Signed-off-by: Stephen Gutekanst <stephen@sourcegraph.com>

Author: Stephen Gutekanst

Taskfile.yml

@@ -182,14 +182,14 @@ tasks:
     desc: "Cross compile to x86_64-linux using Zig"
     cmds:
       - rm -f libs/sinter/bindings/sinter-go/go.mod
-      - cd libs/sinter && zig build -Drelease-fast=true -Dtarget=x86_64-linux
+      - cd libs/sinter && zig build -Drelease-fast=true -Dtarget=x86_64-linux-musl
       - go build -o out/doctree-x86_64-linux ./cmd/doctree/
     env:
       CGO_ENABLED: "1"
       GOOS: "linux"
       GOARCH: "amd64"
-      CC: "zig cc -target x86_64-linux-gnu"
-      CXX: "zig c++ -target x86_64-linux-gnu"
+      CC: "zig cc -target x86_64-linux-musl"
+      CXX: "zig c++ -target x86_64-linux-musl"
     sources:
       - ./**/*.go
       - ./frontend/public/**