make test fails to start on Linux

Nobody has claimed this yet.

I tried running the tests using `make test` today on Linux:

```
⇒ make test
env GO111MODULE=on go run -tags 'libflux' github.com/influxdata/flux/internal/cmd/flux-config --libs --verbose > .cgo_ldflags.tmp
Updating crates.io index
Downloaded chrono v0.4.9
Downloaded derivative v1.0.3
Downloaded walkdir v2.2.9
Downloaded cc v1.0.45
Downloaded maplit v1.0.2
Downloaded serde_json v1.0.41
Downloaded regex v1.3.1
Downloaded flatbuffers v0.6.0
Downloaded serde-aux v0.6.1
Downloaded num-integer v0.1.41
Downloaded num-traits v0.2.8
Downloaded proc-macro2 v0.4.30
Downloaded syn v0.15.44
Downloaded regex-syntax v0.6.12
Downloaded thread_local v0.3.6
Downloaded aho-corasick v0.7.6
Downloaded serde_derive v1.0.101
Downloaded serde v1.0.101
Downloaded smallvec v0.6.10
Downloaded wasm-bindgen v0.2.55
Downloaded bindgen v0.49.2
Downloaded ryu v1.0.2
Downloaded memchr v2.2.1
Downloaded same-file v1.0.5
Downloaded time v0.1.42
Downloaded quote v0.6.13
Downloaded itoa v0.4.4
Downloaded libc v0.2.62
Downloaded clap v2.33.0
Downloaded clang-sys v0.28.1
Downloaded shlex v0.1.1
Downloaded env_logger v0.6.2
Downloaded log v0.4.8
Downloaded cfg-if v0.1.10
Downloaded lazy_static v1.4.0
Downloaded which v2.0.1
Downloaded unicode-xid v0.1.0
Downloaded bitflags v1.2.1
Downloaded syn v1.0.5
Downloaded autocfg v0.1.6
Downloaded proc-macro2 v1.0.5
Downloaded fxhash v0.2.1
Downloaded cexpr v0.3.5
Downloaded quote v1.0.2
Downloaded textwrap v0.11.0
Downloaded byteorder v1.3.2
Downloaded strsim v0.8.0
Downloaded ansi_term v0.11.0
Downloaded termcolor v1.0.5
Downloaded failure v0.1.6
Downloaded unicode-xid v0.2.0
Downloaded unicode-width v0.1.6
Downloaded glob v0.3.0
Downloaded atty v0.2.13
Downloaded vec_map v0.8.1
Downloaded peeking_take_while v0.1.2
Downloaded wasm-bindgen-macro v0.2.55
Downloaded backtrace v0.3.40
Downloaded humantime v1.3.0
Downloaded nom v4.2.3
Downloaded rustc-demangle v0.1.16
Downloaded libloading v0.5.2
Downloaded version_check v0.1.5
Downloaded backtrace-sys v0.1.32
Downloaded wasm-bindgen-macro-support v0.2.55
Downloaded quick-error v1.2.2
Downloaded wasm-bindgen-shared v0.2.55
Downloaded wasm-bindgen-backend v0.2.55
Downloaded bumpalo v2.6.0
Compiling libc v0.2.62
Compiling cc v1.0.45
Compiling cfg-if v0.1.10
Compiling log v0.4.8
Compiling memchr v2.2.1
Compiling proc-macro2 v1.0.5
Compiling lazy_static v1.4.0
Compiling unicode-xid v0.2.0
Compiling autocfg v0.1.6
Compiling syn v1.0.5
Compiling proc-macro2 v0.4.30
Compiling unicode-xid v0.1.0
Compiling glob v0.3.0
Compiling version_check v0.1.5
Compiling byteorder v1.3.2
Compiling serde v1.0.101
Compiling rustc-demangle v0.1.16
Compiling bitflags v1.2.1
Compiling wasm-bindgen-shared v0.2.55
Compiling ryu v1.0.2
Compiling regex-syntax v0.6.12
Compiling quick-error v1.2.2
Compiling unicode-width v0.1.6
Compiling bindgen v0.49.2
Compiling ansi_term v0.11.0
Compiling vec_map v0.8.1
Compiling strsim v0.8.0
Compiling bumpalo v2.6.0
Compiling termcolor v1.0.5
Compiling itoa v0.4.4
Compiling shlex v0.1.1
Compiling syn v0.15.44
Compiling peeking_take_while v0.1.2
Compiling wasm-bindgen v0.2.55
Compiling smallvec v0.6.10
Compiling same-file v1.0.5
Compiling maplit v1.0.2
Compiling thread_local v0.3.6
Compiling humantime v1.3.0
Compiling textwrap v0.11.0
Compiling walkdir v2.2.9
Compiling flatbuffers v0.6.0
Compiling num-traits v0.2.8
Compiling num-integer v0.1.41
Compiling nom v4.2.3
Compiling clang-sys v0.28.1
Compiling quote v0.6.13
Compiling quote v1.0.2
Compiling aho-corasick v0.7.6
Compiling fxhash v0.2.1
Compiling atty v0.2.13
Compiling time v0.1.42
Compiling clap v2.33.0
Compiling cexpr v0.3.5
Compiling backtrace-sys v0.1.32
Compiling libloading v0.5.2
Compiling regex v1.3.1
Compiling backtrace v0.3.40
Compiling failure v0.1.6
Compiling env_logger v0.6.2
Compiling which v2.0.1
Compiling wasm-bindgen-backend v0.2.55
Compiling serde_json v1.0.41
Compiling chrono v0.4.9
Compiling wasm-bindgen-macro-support v0.2.55
Compiling derivative v1.0.3
Compiling serde_derive v1.0.101
Compiling wasm-bindgen-macro v0.2.55
Compiling flux v0.3.0 (/home/edd/.cache/go-build/libflux/@v0.58.0/src/flux)
error: failed to run custom build command for `flux v0.3.0 (/home/edd/.cache/go-build/libflux/@v0.58.0/src/flux)`

Caused by:
process didn't exit successfully: `/home/edd/.cache/go-build/libflux/@v0.58.0/target/release/build/flux-44eec51998873593/build-script-build` (exit code: 101)
--- stdout
cargo:warning=couldn't execute `llvm-config --prefix` (error: No such file or directory (os error 2))
cargo:warning=set the LLVM_CONFIG_PATH environment variable to a valid `llvm-config` executable

--- stderr
thread 'main' panicked at 'Unable to find libclang: "couldn\'t find any valid shared libraries matching: [\'libclang.so\', \'libclang-*.so\', \'libclang.so.*\'], set the `LIBCLANG_PATH` environment variable to a path where one of these files can be found (invalid: [])"', src/libcore/result.rs:1165:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

warning: build failed, waiting for other jobs to finish...
error: build failed
error: ldflags: exit status 101.
exit status 1
make: *** [Makefile:116: .cgo_ldflags] Error 1
```

Start by running `make test` and inspect Makefile:116, where `.cgo_ldflags` invokes `flux-config`. Trace the Rust `flux` build output around `llvm-config` and `LIBCLANG_PATH`; done means the Linux test command gets past this build step and can run the tests.

Written by the indexing model from the issue text.