shell logger: Use fmt functions once

[DrJosh9000]

Description

fmt.Fprintf(w, "%s", fmt.Sprintf(format, v...)) seems silly - and following it with fmt.Fprintln(w) also seems silly.

fmt.Fprintf(w, format+"\n", v...) is faster and does less allocations:

goos: darwin
goarch: arm64
pkg: github.com/buildkite/agent/v3/internal/job/shell
          │ doublefmt.txt │            fmtconcat.txt            │
          │    sec/op     │   sec/op     vs base                │
Printf-10    140.80n ± 0%   87.05n ± 0%  -38.17% (p=0.000 n=30)

          │ doublefmt.txt │           fmtconcat.txt            │
          │     B/op      │   B/op     vs base                 │
Printf-10      40.00 ± 0%   0.00 ± 0%  -100.00% (p=0.000 n=30)

          │ doublefmt.txt │            fmtconcat.txt            │
          │   allocs/op   │ allocs/op   vs base                 │
Printf-10      2.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=30)

Context

I was wondering why two error messages (from different goroutines) collided this way in a job log:

and tracked it to the separate fmt.Fprintln call.

Changes

• Change how format strings are handled in the shell loggers
• Add benchmarks

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)