misc(build): use cross platform sed for devtools script

[paulirish]

Running yarn open-devtools locally, i get an error:

../../front_end/panels/lighthouse/LighthousePanel.ts(266,38): error TS2339: Property 'DOM' does not exist on type 'typeof import("lighthouse/.tmp/chromium-web-tests/devtools/devtools-frontend/out/Default/gen/front_end/third_party/lighthouse/report/report")'.

In #12933 we added a fix for this, but the sed command was invalid.
And since the script didnt have a euo pipefail the script continued despite this happening:

+ npx tsc --allowJs --declaration --emitDeclarationOnly dist/report/bundle.esm.js
+ sed -i '' 's/export type DOM = any;//' dist/report/bundle.esm.d.ts
sed: can't read s/export type DOM = any;//: No such file or directory

(found all this while debugging some CI devtools failures in #13123)

[paulirish]

thanks for looking. it appears this PR is still broken (in two ways). uno momento. :)

[paulirish]

two latest commits explained

unbound variable: https://github.com/GoogleChrome/lighthouse/pull/13123/files/44ecdeca4e3549bdd2e481dbf4dcf50ae7a0ccf0#r733926471

also.. other unbound variable bs: #11331 #11809 #11845

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sed -i BS: https://github.com/GoogleChrome/lighthouse/pull/13123/files/44ecdeca4e3549bdd2e481dbf4dcf50ae7a0ccf0#r733929051