chore: add support for rust 1.48

[rockstar]

Rust 1.48 was released a few days ago. This patch adds the fixes that
will get flux passing in CI again. The following things needed
addressed:

• There is a new lint introduced that checks for push_str on a
  single char str, and recommends using push instead.
• There is an open issue with sccache and rust 1.48. As a result,
  sccache is desabled for now. See sccache v0.2.13 does not work when built with rustc 1.48 mozilla/sccache#887

[codecov-io]

Codecov Report

Merging #3356 (6499c83) into master (9c3cf77) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3356      +/-   ##
==========================================
+ Coverage   47.34%   47.36%   +0.01%     
==========================================
  Files         350      350              
  Lines       35304    35304              
==========================================
+ Hits        16715    16720       +5     
+ Misses      16063    16058       -5     
  Partials     2526     2526              

Impacted Files	Coverage Δ
libflux/go/libflux/parser.go	67.64% <0.00%> (+1.47%)	⬆️
libflux/go/libflux/analyze.go	54.05% <0.00%> (+5.40%)	⬆️

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[drahnr]

I'd recommend using either https://github.com/paritytech/sccache or at least the latest master of mozilla.

[wolffcm]

This looks reasonable to me... I guess builds will be slow until the bug is addressed. Should we have an issue to reenable it? I don't want us to forget.

[rockstar]

I'd recommend using either https://github.com/paritytech/sccache or at least the latest master of mozilla.

I'm not sure I'm comfortable running from master or using an unofficial fork, particularly for something that essentially wraps out compiler. We'll keep an eye on it, and if anything changes, we'll re-consider.

Should we have an issue to reenable it? I don't want us to forget.

There is an XXX comment, but I also filed #3357 to track it.