Support embedded nix builds (#2736)

This adds support for building bpftrace as an appimage. See the
documentation update in nix.md in the commit for usage.

This is intended to replace the current semi-static embedded build.
This approach has both pros and cons over the semi-static embedded build.
For one, the appimage is actually fully static (as it bundles the
necesssary libc). So it's more likely to work on more systems.

The downside is that the appimage is a ~10x larger binary than the
semi-static binary. The appimage also requires fuse to work (which most
systems should support). It's takes ~1s longer to start up which I don't
think is a huge deal.

This closes #2721.

Author: danobi

docs/nix.md

@@ -46,6 +46,17 @@ $ sudo ./result/bin/bpftrace --info 2>&1 | grep LLVM
   LLVM: 13.0.1
 ```
 
+### Build bpftrace as a statically linked binary
+
+```
+$ nix build .#appimage
+$ ldd ./result
+        not a dynamic executable
+$ sudo ./result -e 'BEGIN { print("static!"); exit() }'
+Attaching 1 probe...
+static!
+```
+
 ### Don't use Nix to build, but rather only manage dependencies
 
 ```

flake.lock

@@ -4,9 +4,16 @@
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/release-22.11";
     flake-utils.url = "github:numtide/flake-utils";
+    nix-appimage = {
+      # Use fork until https://github.com/ralismark/nix-appimage/pull/8 is in
+      url = "github:danobi/nix-appimage/5d5093111a1ec4f116c1c57b7a807d41404bfa5e";
+      # Avoid multiple copies of the same dependency
+      inputs.nixpkgs.follows = "nixpkgs";
+      inputs.flake-utils.follows = "flake-utils";
+    };
   };
 
-  outputs = { self, nixpkgs, flake-utils, ... }:
+  outputs = { self, nixpkgs, flake-utils, nix-appimage, ... }:
     # This flake only supports 64-bit linux systems.
     # Note bpftrace support aarch32 but for simplicity we'll omit it for now.
     flake-utils.lib.eachSystem [ "x86_64-linux" "aarch64-linux" ]
@@ -127,6 +134,14 @@
             bpftrace-llvm12 = mkBpftrace pkgs.llvmPackages_12;
             bpftrace-llvm11 = mkBpftrace pkgs.llvmPackages_11;
             bpftrace-llvm10 = mkBpftrace pkgs.llvmPackages_10;
+
+            # Self-contained static binary with all dependencies
+            appimage = nix-appimage.mkappimage.${system} {
+              drv = default;
+              entrypoint = pkgs.lib.getExe default;
+              name = default.name;
+            };
+
           };
 
           # Define apps that can be run with `nix run`