This fixes a bug where `patchShebangs` crashes when trying to patch
files that contain only a shebang (e.g. `#!/bin/bash`) (and nothing
else) and do not end with a newline.
Such file can be produced using `printf "#!/bin/bash" > example` or
`echo -n "#!/bin/bash" > example`.
I don't understand why one would want to create such files, as they do
literally nothing, but the chromium tarball we are using started
shipping some 🫠
Full reproducer:
```nix
with import <nixpkgs> { };
stdenv.mkDerivation {
dontUnpack = true;
name = "patch-shebangs-no-trailing-newline-reproducer";
postPatch = ''
printf "#!/bin/bash" > reproducer
chmod +x reproducer
patchShebangs reproducer
'';
}
```
```
❯ nix-build reproducer.nix
this derivation will be built:
/nix/store/vmbshdkdk4a0bayw3wi21wvxyhzpcsy2-patch-shebangs-no-trailing-newline-reproducer.drv
building '/nix/store/vmbshdkdk4a0bayw3wi21wvxyhzpcsy2-patch-shebangs-no-trailing-newline-reproducer.drv'...
patching sources
patching script interpreter paths in reproducer
/nix/store/vr6wwdxkmyy44sg0gwxi10b8fc5zhwz0-stdenv-linux/setup: line 144: pop_var_context: head of shell_variables not a function context
error: builder for '/nix/store/vmbshdkdk4a0bayw3wi21wvxyhzpcsy2-patch-shebangs-no-trailing-newline-reproducer.drv' failed with exit code 1;
last 3 log lines:
> patching sources
> patching script interpreter paths in reproducer
> /nix/store/vr6wwdxkmyy44sg0gwxi10b8fc5zhwz0-stdenv-linux/setup: line 144: pop_var_context: head of shell_variables not a function context
For full logs, run 'nix log /nix/store/vmbshdkdk4a0bayw3wi21wvxyhzpcsy2-patch-shebangs-no-trailing-newline-reproducer.drv'.
```
This change adds a flag to update shebang paths that point to the Nix
store. This is particularly useful when a cross-compiled package uses
same script at compile-time and run-time, but the interpreter must be
changed since hostPlatform != buildPlatform.
